Integrated switchless programmable attenuator and low noise amplifier

ABSTRACT

An integrated receiver with channel selection and image rejection substantially implemented on a single CMOS integrated circuit is described. A receiver front end provides programmable attenuation and a programmable gain low noise amplifier. Frequency conversion circuitry advantageously uses LC filters integrated onto the substrate in conjunction with image reject mixers to provide sufficient image frequency rejection. Filter tuning and inductor Q compensation over temperature are performed on chip. The filters utilize multi track spiral inductors. The filters are tuned using local oscillators to tune a substitute filter, and frequency scaling during filter component values to those of the filter being tuned. In conjunction with filtering, frequency planning provides additional image rejection. The advantageous choice of local oscillator signal generation methods on chip is by PLL out of band local oscillation and by direct synthesis for in band local oscillator. The VCOs in the PLLs are centered using a control circuit to center the tuning capacitance range. A differential crystal oscillator is advantageously used as a frequency reference. Differential signal transmission is advantageously used throughout the receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication Nos. 60/108,459, 60/108,209, 60/108,210 filed Nov. 12, 1998;U.S. Provisional Application No. 60/117,609 filed Jan. 28, 1999; U.S.Provisional Application Nos. 60/136,115 and 60/136,116 filed May 26,1999; U.S. Provisional Application No. 60/136,654 filed May 27, 1999;and U.S. Provisional Application No. 60/159,726 filed Oct. 15, 1999; thecontents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This application relates generally to receiver circuits and, inparticular to a CATV tuner with a frequency plan and architecture thatallows the entire receiver, including the filters, to be integrated ontoa single integrated circuit.

BACKGROUND OF THE INVENTION

[0003] Radio receivers, or tuners, are widely used in applicationsrequiring the reception of electromagnetic energy. Applications caninclude broadcast receivers such as radio and television, set top boxesfor cable television, receivers in local area networks, test andmeasurement equipment, radar receivers, air traffic control receivers,and microwave communication links among others. Transmission of theelectromagnetic energy may be over a transmission line or byelectromagnetic radio waves.

[0004] The design of a receiver is one of the most complex design tasksin electrical engineering. In the current state of the art, there aremany design criteria that must be considered to produce a working radioreceiver. Tradeoffs in the design's performance are often utilized toachieve a given objective. There are a multitude of performancecharacteristics that must be considered in designing the receiver.However, certain performance characteristics are common to allreceivers. Distortion and noise are two such parameters. The process ofcapturing the signal creates distortion that must be accounted for inthe design of the radio receiver. Once a radio signal is captured, thenoise surrounding the received signal in the receiver must beconsidered. Radio signals are often extremely weak and if noise ispresent in the circuit, the signal, even though satisfactorily received,can be easily lost in this noise floor. The current state of the art inreceiver design is often directed to overcoming these receiverlimitations in a cost effective manner.

SUMMARY OF THE INVENTION

[0005] There is therefore provided in a present embodiment of theinvention a receiver front end circuit that is integrated into asemiconductor substrate that provides a differential signal output. thereceiver front end contains a programable attenuator having a resistiveladder network. The network has multiple tap points, so that a differentvalue of attenuation may be selected by making contact with the tappoints. The tap points are connected by activating a low noisedifferential pair amplifier connected to each tap point. Thedifferential pair amplifiers are selectively turned on, signally or as agroup, to amplify the previously attenuated signal. The resultant signalis amplified or attenuated depending upon tap points connected. Theoutput is a differential signal of low noise.

[0006] Many of the attendant features of this invention will be morereadily appreciated as the same becomes better understood by referenceto the following detailed description considered in connection with theaccompanying drawings, in which like reference symbols designate likeparts throughout.

DESCRIPTION OF THE DRAWINGS

[0007] These and other features and advantages of the present inventionwill be better understood from the following detailed description readin light of the accompanying drawings, wherein

[0008]FIG. 1 is an illustration of a portion of the over-the-airbroadcast spectrum allocations in the United States;

[0009]FIG. 2 is an illustration of the frequency spectrum of harmonicdistortion products;

[0010]FIG. 3 is an illustration of a spectrum of even and odd orderintermodulation distortion products;

[0011]FIG. 4 is an illustration of interference caused at the IFfrequency by a signal present at the image frequency;

[0012]FIG. 5 is an illustration of a typical dual conversion receiverutilizing an up conversion and a subsequent down conversion;

OSCILLATOR FIGS.

[0013]FIG. 6 is a semi-schematic simplified timing diagram ofdifferential signals, including a common mode component, as might bedeveloped by a differential crystal oscillator in accordance with theinvention;

[0014]FIG. 7 is a semi-schematic block diagram of a differential crystaloscillator, including a quartz crystal resonator and oscillator circuitdifferentially coupled to a linear buffer amplifier in accordance withthe invention;

[0015]FIG. 8 is a simplified schematic illustration of differentialsignals present at the output of a crystal resonator;

[0016]FIG. 9 is a simplified schematic diagram of a quartz crystalresonator equivalent circuit;

[0017]FIG. 10 is a simplified graphical representation of a plot ofimpedance vs. frequency for a crystal resonator operating nearresonance;

[0018]FIG. 11 is a simplified graphical representation of a plot ofphase vs. frequency for a crystal resonator operating near resonance;

[0019]FIG. 12 is a simplified schematic diagram of the differentialoscillator circuit of FIG. 7;

[0020]FIG. 13 is a simplified, semi-schematic block diagram of aperiodic signal generation circuit including a crystal oscillator havingbalanced differential outputs driving cascaded linear and non-linearbuffer stages;

[0021]FIG. 14 is a simplified schematic diagram of a differential foldedcascade linear amplifier suitable for use in connection with the presentinvention;

[0022]FIG. 15 is a simplified, semi-schematic diagram of a differentialnonlinear buffer amplifier suitable for use as a clock buffer inaccordance with the invention;

[0023]FIG. 16 is a semi-schematic illustration of an alternativeembodiment of the differential oscillator driver circuit;

[0024]FIG. 17 is an block diagram of a differential crystal oscillatoras a reference signal generator in a phase-lock-loop; and

[0025]FIG. 18 is a simplified block diagram of an illustrative frequencysynthesizer that might incorporate the differential periodic signalgeneration circuit of the invention.

COARSE/FINE PLL TUNING FIGURES

[0026]FIG. 19 is a block diagram illustrating the exemplary frequencyconversions for receiver tuning utilized in the embodiments of theinvention;

[0027]FIG. 20 is a block diagram of an exemplary tuner designed toreceive a 50 to 860 MHz bandwidth containing a multiplicity of channels;

[0028]FIG. 21 is an exemplary table of frequencies utilizing coarse andfine PLL tuning to derive a 44 MHz IF;

[0029]FIG. 22 is an illustration of an alternative embodiment of thecoarse and fine PLL tuning method to produce an exemplary final IF of 36MHz;

[0030]FIG. 23 is a block diagram of a dummy component used to model anoperative component on an integrated circuit chip;

FILTER TUNING FIGURES

[0031]FIG. 24a is a block diagram of a tuning process,

[0032]FIG. 24b is a flow diagram of the tuning process, and FIG. 24c isan exemplary illustration of the tuning process;

[0033]FIG. 25 is a block diagram of an exemplary tuning circuit;

[0034]FIG. 26 illustrates the amplitude and phase relationship in an LCfilter at resonance;

[0035]FIG. 27 is a schematic diagram showing the configuration ofswitchable capacitors in a differential signal transmission embodiment;

INDUCTOR Q TEMPERATURE COMPENSATION FIGURES

[0036]FIG. 28 is an illustration of a typical spiral inductor suitablefor integrated circuit applications;

[0037]FIG. 29 is an illustration of the effect of decreasing “Q” on theselectivity of a tuned circuit;

[0038]FIG. 30 is an illustration of a typical filter bank utilized inembodiments of the invention for filtering I and Q IF signals;

[0039]FIG. 31 is a diagram of a transconductance stage with an LC load;

[0040]FIG. 32 shows a transconductance stage with an LC load and Qenhancement;

[0041]FIG. 33 shows a method of tuning inductor Q over temperature;

COMMUNICATIONS RECEIVER FIGURES

[0042]FIG. 34 is a block diagram of a communications network utilizing areceiver according to any one of the exemplary embodiments of theinvention;

RECEIVER FRONT END-PROGRAMABLE ATTENUATOR AND LNA FIGURES

[0043]FIG. 35 is an is an illustration of the input and output signalsof the integrated switchless programmable attenuator and low noiseamplifier;

[0044]FIG. 36 is a functional block diagram of the integrated switchlessprogrammable attenuator and low noise amplifier circuit;

[0045]FIG. 37 is a simplified diagram showing the connection of multipleattenuator sections to the output of the integrated switchlessprogrammable attenuator and low noise amplifier;

[0046]FIG. 38 is an illustration of an exemplary embodiment showing howthe attenuator can be removed from the circuit so that only the LNAs areconnected;

[0047]FIG. 39 is an attenuator circuit used to achieve one dB per stepattenuation;

[0048]FIG. 40 is an exemplary embodiment of an attenuator for achievinga finer resolution in attenuation then shown in FIG. 5;

[0049]FIG. 41 is an illustration of the construction of series andparallel resistors used in the attenuator circuit of the integratedswitchless programmable attenuator and low noise amplifier;

[0050]FIG. 42 is an illustration of a preferred embodiment utilized toturn on current tails of the differential amplifiers;

[0051]FIG. 43 is an illustration of an embodiment showing how theindividual control signals used to turn on individual differential pairamplifiers are generated from a single control signal;

[0052]FIG. 44 is an illustration of an embodiment of comparatorcircuitry used to activate individual LNA amplifier stages;

LOCAL OSCILLATOR GENERATION FIGURES

[0053]FIG. 45 is a block diagram illustrating the exemplary generationof the local oscillator signals utilized in the embodiments of theinvention;

NARROW BAND VCO TUNING FIGURES

[0054]FIG. 46 is a schematic of a PLL having its VCO controlled by anembodiment of a VCO tuning control circuit;

[0055]FIG. 47 is a process flow diagram illustrating the process oftuning the VCO with an embodiment of a VCO control circuit;

RECEIVER FIGURES

[0056]FIG. 48 is a block diagram of the first exemplary embodiment ofthe invention;

[0057]FIG. 49 is an illustration of the frequency planning utilized inthe exemplary embodiments of the invention;

[0058]FIG. 50 is a block diagram showing how image frequencycancellation is achieved in an I/Q mixer;

[0059]FIG. 51 is a block diagram of the second exemplary embodiment ofthe present invention;

[0060]FIG. 52 is a block diagram of the third exemplary embodiment ofthe present invention;

[0061]FIG. 53 is a block diagram of a CATV tuner that incorporates thefully integrated tuner architecture; and

TELEPHONY OVER CABLE EMBODIMENT FIGURE

[0062]FIG. 54 is a block diagram of a low power embodiment of thereceiver that has been configured to receive cable telephony signals.

ELECTRONIC CIRCUITS INCORPORATING EMBODIMENTS OF THE RECEIVER FIGURES

[0063]FIG. 55 is a block diagram of a set top box that incorporates thereceiver embodiments;

[0064]FIG. 56 is a block diagram of a television that incorporates thereceiver embodiments;

[0065]FIG. 57 is a block diagram of a VCR that incorporates the receiverembodiments; and

[0066]FIG. 58 is a block diagram of a cable modem that incorporates theintegrated switchless programmable attenuator and low noise amplifier.

DETAILED DESCRIPTION OF THE INVENTION

[0067]FIG. 1 is an illustration of a portion of the radio frequencyspectrum allocations by the FCC. Transmission over a given media occursat any one of a given range of frequencies that are suitable fortransmission through a medium. A set of frequencies available fortransmission over a medium are divided into frequency bands 102.Frequency bands are typically allocations of frequencies for certaintypes of transmission. For example FM radio broadcasts, FM being a typeof modulation, is broadcast on the band of frequencies from 88 MHz to108 MHz 104. Amplitude modulation (AM), another type of modulation, isallocated the frequency band of 540 kHz to 1,600 kHz 106. The frequencyband for a type of transmission is typically subdivided into a number ofchannels. A channel 112 is a convenient way to refer to a range offrequencies allocated to a single broadcast station. A stationbroadcasting on a given channel may transmit one or more radio frequency(RF) signals within this band to convey the information of a broadcast.Thus, several frequencies transmitting within a given band may be usedto convey information from a transmitter to a broadcast receiver. Forexample, a television broadcast channel broadcasts its audio signal(s)108 on a frequency modulated (FM) carrier signal within the givenchannel. A TV picture (P) 110 is a separate signal broadcast using atype of amplitude modulation (AM) called vestigial side band modulation(VSB), and is transmitted within this channel.

[0068] In FIG. 1 channel allocations for a television broadcast bandshowing the locations of a picture and a sound carrier frequencieswithin a channel are shown. Each channel 112 for television has anallocated fixed bandwidth of 6 MHz. The picture 110 and sound 108carriers are assigned a fixed position relative to each other within the6 MHz band. This positioning is not a random selection. The picture andsound carriers each require a predetermined range of frequencies, or abandwidth (BW) to sufficiently transmit the desired information. Thus, achannel width is a fixed 6 MHz, with the picture and sound carrierposition fixed within that 6 MHz band, and each carrier is allocated acertain bandwidth to transmit its signal.

[0069] In FIG. 1 it is seen that there are gaps between channels 114,and also between carrier signals 116. It is necessary to leave gaps ofunused frequencies between the carriers and between the channels toprevent interference between channels and between carriers within agiven channel. This interference primarily arises in the receivercircuit that is used to receive these radio frequency signals, convertthem to a usable frequency, and subsequently demodulate them.

[0070] Providing a signal spacing allows the practical design andimplementation of a receiver without placing unrealistic requirements onthe components in the receiver. The spaces help prevent fluctuations inthe transmission frequency or spurious responses that are unwantedbyproducts of the transmission not to cause interference and signaldegradation within the receiver. Also, signal spacing allows the designrequirements of frequency selective circuits in the receiver to berelaxed, so that the receiver may be built economically while stillproviding satisfactory performance. These spectrum allocations andspacings were primarily formulated when the state of the art in receiverdesign consisted of discrete components spaced relatively far apart on aprinted circuit board. The increasing trend towards miniaturization haschallenged these earlier assumptions. The state of the art in integratedcircuit receiver design has advanced such that satisfactory performancemust be achieved in light of the existing spectrum allocations andcircuit component crowding on the integrated circuit. New ways ofapplying existing technology, as well as new technology are continuallybeing applied to realize a miniaturized integrated receiver thatprovides satisfactory performance. Selectivity is a principal measure ofreceiver performance. Designing for sufficient selectivity not onlyinvolves rejecting other channels, but the rejection of distortionproducts that are created in the receiver or are part of the receivedsignal. Design for minimization or elimination of spurious responses isa major objective in state of the art receiver design.

[0071]FIG. 2 is an illustration of harmonic distortion products.Transmitted spurious signals, and spurious signals generated in areceiver, most commonly consist of harmonics created by one frequencyand intermodulation distortion, created by the interaction of multiplefrequencies. Spurious signals at other than the desired frequency arisefrom the inherent nonlinear properties in the circuit components used.These nonlinearities can not be eliminated, but by careful engineeringthe circuitry can be designed to operate in a substantially linearfashion.

[0072] When a single frequency called a fundamental 202 is generated,unwanted spurious signals 204 are always generated with thisfundamental. The spurious signals produced as a result of generating asingle frequency (f) 202 are called harmonics 204 and occur at integermultiples of the fundamental frequency (2f, 3f, . . . ) The signalstrength or amplitude of these harmonics decrease with increasingharmonic frequency. Fortunately these distortion products fall one ormore octaves away from the desired signal, and can usually besatisfactorily filtered out with a low pass filter that blocks allfrequencies above a pre-selected cut-off frequency. However, if thereceiver is a wide band or multi octave bandwidth receiver, theseharmonics will fall within the bandwidth of the receiver and cannot below pass filtered, without also filtering out some of the desiredsignals. In this case, other methods known to those skilled in the art,such as reducing the distortion products produced, must be used toeliminate this distortion.

[0073] Radio signals do not exist in isolation. The radio frequencyspectrum is populated by many channels within a given band transmittingat various frequencies. When a radio circuit is presented with two ormore frequencies, these frequencies interact, or intermodulate, tocreate distortion products that occur at known frequency locations.

[0074]FIG. 3 is an illustration of intermodulation distortion products.Whenever two or more frequencies are present they interact to produceadditional spurious signals that are undesired. FIG. 3 illustrates aspurious response produced from the interaction of two signals, f₁ 302and f₂ 304. This particular type of distortion is called intermodulationdistortion (IMD). These intermodulation distortion products 306 areassigned orders, as illustrated. In classifying the distortion the IMproducts are grouped into two families, even and odd order IM products.Odd order products are shown in FIG. 3.

[0075] In a narrow band systems the even order IM products can be easilyfiltered out, like harmonics, because they occur far from the twooriginal frequencies. The odd order IM products 306 fall close to thetwo original frequencies 302, 304. In a receiver these frequencies wouldbe two received signals or a received channel and a local oscillator.These products are difficult to remove. The third order products 306 arethe most problematic in receiver design because they are typically thestrongest, and fall close within a receiver's tuning band close to thedesired signal. IM distortion performance specifications are importantbecause they are a measure of the receiver's immunity to strong out ofband signal interference.

[0076] Third order products 308 occur at (f₁−Δf) and at (f₂+Δf), whereΔf=f₂=f₁. These unwanted signals may be generated in a transmitter andtransmitted along with desired signal or are created in a receiver.Circuitry in the receiver is required to block these signals. Theseunwanted spurious responses arise from nonlinearities in the circuitrythat makes up the receiver.

[0077] The circuits that make up the receiver though nonlinear arecapable of operating linearly if the signals presented to the receivercircuits are confined to signal levels within a range that does not callfor operation of the circuitry in the nonlinear region. This can beachieved by careful design of the receiver.

[0078] For example, if an amplifier is over driven by signals presentedto it greater than it was designed to amplify, the output signal will bedistorted. In an audio amplifier this distortion is heard on a speaker.In a radio receiver the distortion produced in nonlinear circuits,including amplifiers and mixers similarly causes degradation of thesignal output of the receiver. On a spectrum analyzer this distortioncan be seen; levels of the distortion increase to levels comparable tothe desired signal.

[0079] While unwanted distortion such as harmonic distortion, can befiltered out because the harmonics most often fall outside of thefrequency band received, other distortion such as inter-modulationdistortion is more problematic. This distortion falls within a receivedsignal band and cannot be easily filtered out without blocking otherdesired signals. Thus, frequency planning is often used to control thelocation of distortion signals that degrade selectivity.

[0080] Frequency planning is the selection of local oscillator signalsthat create the intermediate frequency (IF) signals of the downconversion process. It is an analytical assessment of the frequenciesbeing used and the distortion products associated with these frequenciesthat have been selected. By evaluating the distortion and its strength,an engineer can select local oscillator and IF frequencies that willyield the best overall receiver performance, such as selectivity andimage response. In designing a radio receiver, the primary problemsencountered are designing for sufficient sensitivity, selectivity andimage response.

[0081] Selectivity is a measure of a radio receiver's ability to rejectsignals outside of the band being tuned by a radio receiver. A way toincrease selectivity is to provide a resonant circuit after an antennaand before the receiver's frequency conversion circuitry in a “frontend.” For example, a parallel resonant circuit after an antenna andbefore a first mixer that can be tuned to the band desired will producea high impedance to ground at the center of the band. The high impedancewill allow the antenna signal to develop a voltage across thisimpedance. Signals out of band will not develop the high voltage and arethus attenuated.

[0082] The out of band signal rejection is determined by a qualityfactor or “Q” of components used in the resonant circuit. The higher theQ of a circuit in the preselector, the steeper the slope of theimpedance curve that is characteristic of the preselector will be. Asteep curve will develop a higher voltage at resonance for signals inband compared to signals out of band. For a resonant circuit with low Qa voltage developed across the resonant circuit at a tuned frequencyband will be closer in value to the voltage developed across theresonant circuit out of band. Thus, an out of band signals would becloser in amplitude to an in band signals than if a high Q circuit wereconstructed.

[0083] This type of resonant circuit used as a preselector will increasefrequency selectivity of a receiver that has been designed with thisstage at its input. If an active preselector circuit is used between anantenna and frequency conversion stages, the sensitivity of the receiverwill be increased as well as improving selectivity. If a signal is weakits level will be close to a background noise level that is present onan antenna in addition to a signal. If this signal cannot be separatedfrom the noise, the radio signal will not be able to be converted to asignal usable by the receiver. Within the receiver's signal processingchain, the signal's amplitude is decreased by losses at every stage ofthe processing. To make up for this loss the signal can be amplifiedinitially before it is processed. Thus, it can be seen why it isdesirable to provide a circuit in the receiver that provides frequencyselectivity and gain early in the signal processing chain.

[0084] Radio frequency tuners are increasingly being designed with majorportions of their circuitry implemented as an integrated circuit. In thestate of the art to minimize distortion products created in thereceiver, exotic materials such as gallium arsenide (GaAs) are used. Areceiver implemented on this type of material will typically have lowerdistortion and noise present than in a similarly constructed receiverconstructed on silicon. Silicon, is an attractive material due to itslow cost. In addition, a CMOS circuit implemented on silicon has theadditional benefit of having known processing characteristics that allowa high degree of repeatability from lot to lot of wafers. The state ofthe art has not achieved a completely integrated receiver in CMOScircuitry. A reason for this is the difficulty of eliminating receiverdistortion and noise.

[0085] The distortion products discussed above that are created in thereceiver can, in the majority of cases, also be reduced by setting anappropriate drive level in the receiver, and by allowing a sufficientspacing between carriers and channels. These receiver design parametersare dependent upon many other factors as well, such as noise present inthe system, frequency, type of modulation, and signal strength amongothers. Noise is one of the most important of these other parametersthat determines the sensitivity of the receiver, or how well a weaksignal may be satisfactorily received.

[0086] Noise is present with the transmitted signal, and also generatedwithin a receiver. If excessive noise is created in a receiver a weaksignal may be lost in a “noise floor”. This means that the strength ofthe received signal is comparable to the strength of the noise present,and the receiver is incapable of satisfactorily separating a signal outof this background noise, or floor. To obtain satisfactory performance a“noise floor” is best reduced early in a receiver's chain of circuitcomponents.

[0087] Once a signal is acquired and presented to a receiver, inparticularly an integrated receiver with external pins, additional noisemay be radiated onto those pins. Thus, additional added noise at thereceiver pins can degrade the received signal.

[0088] In addition to the noise that is present on an antenna or a cableinput to a receiver, noise is generated inside the radio receiver. At aUHF frequency range this internal noise predominates over the noisereceived with the signal of interest. Thus, for the higher frequenciesthe weakest signal that can be detected is determined by the noise levelin the receiver. To increase the sensitivity of the receiver a“pre-amplifier” is often used after an antenna as a receiver front endto boost the signal level that goes into the receiver. This kind ofpre-amplification at the front end of the amplifier will add noise tothe receiver due to the noise that is generated inside of this amplifiercircuit. However, the noise contribution of this amplifier can beminimized by using an amplifier that is designed to produce minimalnoise when it amplifies a signal, such as an LNA. Noise does not simplyadd from stage to stage; the internal noise of the first amplifiersubstantially sets the noise floor for the entire receiver.

[0089] In calculating a gain in a series of cascaded amplifiers theoverall gain is simply the sum of the gains of the individual amplifiersin decibels. For example, the total gain in a series of two amplifierseach having a gain of 10 dB is 20 dB for a overall amplifier. Noisefloor is commonly indicated by the noise figure (NF). The larger the NFthe higher the noise floor of the circuit.

[0090] A Cascaded noise figure is not as easily calculated as amplifiergain; its calculation is non-intuitive. In a series of cascadedamplifiers, gain does not depend upon the positioning of the amplifiersin the chain. However, in achieving a given noise figure for a receiver,the placement of the amplifiers is critical with respect to establishinga receiver's noise floor. In calculating the noise figure for anelectronic system Friis' equation is used to calculate the noise figureof the entire system. Friis' equation is $\begin{matrix}{{N\quad F_{t\quad o\quad t\quad a\quad l}} = {{N\quad F_{1}} + \frac{{N\quad F_{2}} - 1}{G_{1}} + \frac{{N\quad F_{3}} - 1}{G_{1}G_{2}} + \ldots + \frac{{N\quad F_{n}} - 1}{G_{1}G_{2}\quad \ldots \quad G_{n}}}} & (1)\end{matrix}$

[0091] NF_(total)=system noise figure

[0092] NF₁=noise figure of stage-1

[0093] NF₂=noise figure of stage-2

[0094] NF_(n)=noise figure of stage-nth

[0095] G₁=gain of stage-1

[0096] G₂=gain of stage-2

[0097] G_(N)=gain of nth stage

[0098] What can be seen from this equation is that the noise figure of afirst stage is the predominant contributor to a total noise figure. Forexample, the noise figure of a system is only increased a small amountwhen a second amplifier is used. Thus, it can be seen that the noisefigure of the first amplifier in a chain of amplifiers or systemcomponents is critical in maintaining a low noise floor for an entiresystem or receiver. A low NF amplifier typically requires a low noisematerial for transistors, such as gallium arsenide. Later amplifiersthat do not contribute significantly to the noise, are constructed of acheaper and noisier material such as silicon.

[0099] The initial low noise amplifiers are typically constructed fromexpensive materials such as gallium arsenide to achieve sufficientperformance. Gallium arsenide requires special processing, furtheradding to its expense. Additionally, GaAs circuits are not easilyintegrated with silicon circuits that make up the bulk of the receiversin use. It would be desirable to achieve identical performance with aless costly material, such as silicon. Silicon requires less costlyprocessing. Further it is advantageous if a standard process, such asCMOS, could be used to achieve the required low noise design. Given thetrend towards miniaturization and high volume production, it is highlydesirable to be able to produce an integrated receiver with a low noisefloor on silicon.

[0100] Within a receiver the layout and spacing of circuitry is criticalto avoid the injection of noise generated in other portions of thecircuit onto a received signal. If a tuner is placed on a semiconductorsubstrate noise generated in the substrate itself will interfere with,and degrade the received signal, this has been a problem preventingcomplete integration of a receiver on silicon.

[0101] Historically low noise substrates, fabricated from exotic andcostly materials such as gallium arsenide have been used to reduce noisegenerated by the semiconductor substrate. However, it would beadvantageous to be able to fabricate a receiver on a single CMOSsubstrate. CMOS advantageously is a known process that may beimplemented economically for volume production. Currently a receiverfabricated completely in CMOS has not been available without utilizingexternal components in the received signal path. Each time the signal isrouted on or off of the integrated circuit additional opportunities forthe introduction of noise into a signal path are provided. Minimizingthis introduction of noise is an ongoing problem in receiver design.

[0102] After preselection and low noise amplification that is performedin a front end of a receiver, the signal next enters the receiver'sfrequency conversion circuitry. This circuitry takes channels that havebeen passed through the front end and converts one of the selectedchannel's frequencies down to one or more known frequencies (f_(IF) orIFs) . This frequency conversion is accomplished through the use of acircuit called a mixer that utilizes a local oscillator signal (f_(LO)),usually generated in the receiver, to tune a received channel to an IFfrequency while blocking the other channels. Spurious signals,previously described, are produced in this receiver circuitry, and anadditional problem known as “image response” is encountered that must beconsidered in the receiver's design.

[0103] It is well known to those skilled in the art that when twosinusoidal signals of differing frequencies are multiplied together bytheir application to a nonlinear device, such as a mixer, that signalsof a differing frequency are produced. A mixer has three ports: f_(RF)receives a low level radio frequency signal that contains the desiredmodulation, f_(LO) is a high level signal from a local oscillator, andf_(IF) is the resultant mixer product or intermediate frequencyproduced. These frequencies are related:

f _(IF) =mf _(RF) ±nf _(LO)  (2)

[0104] where

[0105] m=0, 1, 2, 3, . . . and

[0106] n=0, 1, 2, 3, . . .

[0107] In a typical first order circuit (m=n=1) four frequencies areproduced: f_(RF), f_(LO), f_(IFLO)=f_(RF)−f_(LO) andf_(IFHI)=f_(RF)+f_(LO). A f_(IFLO) and f_(IFHI) being termedintermediate frequencies. In receivers the common practice is to selecteither the sum or difference IF frequency by filtering out the undesiredone. Since both signals contain the same information, only one is neededin the subsequent circuitry.

[0108] One or more mixers are advantageously used in radio receivers toconvert a high frequency radio signal which is received into a lowerfrequency signal that can be easily processed by subsequent circuitry.Mixers are also used to tune multiple channels, so that different tunedcircuits are not required for each channel. By changing a localoscillator frequency, differing radio frequencies received can be tunedto produce a constant intermediate frequency value regardless of thefrequency of the received channel. This means that circuit componentsused to process the intermediate frequency may be fixed in value, withno tuning of capacitors or coils required. Thus, circuits in an IF stripare all fixed-tuned at an IF frequency. A receiver constructed in thismanner, using one or more frequency conversions, is called asuperheterodyne radio receiver.

[0109] A disadvantage of a superheterodyne radio receiver is that any ofthe one or more local oscillators within the receiver also acts as aminiature transmitter. A receiver “front end” alleviates this problem byisolating an antenna from the remaining receiver circuitry.

[0110] By positioning a radio frequency amplifier between the antennaand the frequency converting stages of a receiver, additional isolationbetween the receiver circuitry and the antenna is achieved. The presenceof an amplifier stage provides attenuation for any of the one or morelocal oscillator signals from the frequency conversion stages that areradiated back towards the antenna or a cable distribution network. Thisincreased isolation has the benefit of preventing radiation of a localoscillator signal out the antenna which could cause radio frequencyinterference from a local oscillator. If radiated these and othersignals present could create interference in another receiver present atanother location.

[0111]FIG. 4 is an illustration that shows an image frequency's 402relation to other signals present 404, 406, 408 at a mixer. Imagefrequency suppression is an important parameter in a receivers design.In a radio receiver two frequencies input to a radio receiver 404, 406will yield a signal at the IF frequency 408. A receiver willsimultaneously detect signals at the desired frequency 404 and also anysignals present at an undesired frequency known as the image frequency402. If there is a signal present at the image frequency, it willtranslate down to the IF frequency 408 and cause interference with thereception of the desired channel. Both of these signals will beconverted to the IF frequency unless the receiver is designed to preventthis. The image frequency 402 is given by:

f _(I) =f _(RF)+2f _(IF)  (3)

[0112] where f_(I) is the image frequency. This is illustrated in FIG.4. A frequency that is spaced the IF frequency 410 below the localoscillator frequency (f_(RF)) 404, and a frequency that is spaced theintermediate frequency 412 above the local oscillator signal (f_(I))402, will both be converted down to the intermediate frequency (f_(IF))408. The usual case is that a frequency that occurs lower than the localoscillator signal is the desired signal. The signal occurring at thelocal oscillator frequency plus the intermediate frequency 402 is anunwanted signal or noise at that frequency that is converted to the IFfrequency causing interference with the desired signal.

[0113] In FIG. 4 the exemplary 560 KHz signal 404 is a radio stationthat the tuner is tuned to receive. The exemplary 1470 KHz signal 402 isanother radio station transmitting at that particular frequency. If adesigner of the receiver had picked an exemplary local oscillator signalof 1015 KHz 406 then both of these radio stations would besimultaneously converted to an exemplary IF frequency of 455 KHz 408.The person listening to the radio would simultaneously hear both radioprograms coming out of his speaker. This illustrates the need for thecareful selection of local oscillator frequencies when designing a radioreceiver. The selection of local oscillator frequencies is a part offrequency planning and used by those skilled in the art to design areceiver that will provide frequency conversions needed with minimaldistortion.

[0114]FIG. 5 illustrates a dual (or double) conversion receiver 502.Such a multiple conversion receiver allows selectivity, distortion andstability to be controlled through a judicious frequency planning. Inthe double conversion receiver 502 a received signal 504 is first mixed506 to a first intermediate frequency, and then mixed 508 down to asecond intermediate frequency. In this type of receiver the first IFfrequency is made to be high so that a good image rejection is achieved.The second IF is made low so that good adjacent channel selectivity isachieved.

[0115] If the first IF frequency is low an image frequency falls higherin frequency, or closer to the center of a pass band of an RFselectivity curve of a receiver “front end,” 510 and undergoes littleattenuation. If the IF frequency is high the image frequency falls fardown on the skirt of the RF selectivity curve for the receiver “frontend” receiving a required attenuation. Thus, the selectivity of thereceiver acts to attenuate the image frequency when a high IF frequencyis used. As an added benefit a high image frequency provides less of achance for interference from a high powered station. This is because athigher frequencies transmitted power is often lower due to thedifficulties in generating RF power as frequency increases.

[0116] A low second IF frequency produces a good adjacent channelselectivity. Frequency spacing between adjacent channels is fixed. Toprevent interference from adjacent channels the receiver must possess agood selectivity. Selectivity can be achieved through a RF tunedcircuit, and more importantly by the superior selectivity provided by afrequency conversion process. The selectivity improvement given by usinga low IF is shown by considering a percent separation of a desired andan undesired signal relative to total signal bandwidth. If a separationbetween the desired and undesired signals is constant a second IF signalfalling at the lower frequency will give a larger percent separationbetween the signals. As a result it is easier to distinguish between IFsignals that are separated by a larger percentage of bandwidth. Thus,the judicious selection of two intermediate frequencies in a doubleconversion receiver is often used to achieve a given design goal, suchas image frequency rejection and selectivity.

[0117] Additionally, the use of a second IF frequency allows gain in thereceiver to be distributed evenly. Distributing gain helps preventinstability in the receiver. Instability usually is seen as anoscillating output signal 512. Distributing the gain among several IFamplifiers 514, 516, 518 reduces the chance of this undesirable effect.Often to further distribute the gain required in a system design a thirdfrequency conversion, and a third IF frequency, will be utilized.

[0118] After a receiver front end that possibly contains a low noiseamplifier, additional amplifiers are often seen in the various IFstrips. An amplifier in an IF strip does not require frequency tuningand provides signal gain to make up for signal losses, encountered inprocessing a received signal. Such losses can include conversion loss inmixers and the insertion loss encountered by placing a circuit element,such as a filter or an isolator in the IF strip.

[0119] In receivers filters are used liberally to limit unwantedfrequencies that have been escaped previous elimination in a “frontend,” or to eliminate unwanted frequencies that have been createdimmediately preceding a filter. In addition to attenuating unwantedfrequencies, a desired signal will also undergo some attenuation. Thisattenuation results from an insertion loss of a filter, or some othercomponent, and if uncompensated, will degrade a signal. This isespecially true when a series of filters are cascaded, since the effectis additive.

[0120] Often a series of multiple filters are cascaded in a given IFstrip. These filters typically have an identical responsecharacteristic. The cascaded filters are used to increase theselectivity of the receiver. While it is true that the insertion loss inthe pass band is the sum of individual filter insertion losses, asmeasured in decibels, a rejection improvement obtained outside of thepass band is the sum of the rejections at the given frequency. Thus,three cascaded filters, each having an insertion loss of 0.01 dB at acenter frequency, would have a total insertion loss of 0.03 dB. If therejection in the stop band, a given frequency away from the centerfrequency of the filter, were 20 dB, then a total rejection for 3cascaded filters would be 60 dB, a great improvement in filterselectivity.

[0121] In choosing intermediate frequencies for IF strips in thereceiver, no concrete design guidelines exist. Also because of a widevariance in design goals that are encountered in receiver design,concrete methodologies do not exist. Each receiver must be uniquelyengineered to satisfy a series of system design goals taking intoconsideration design tradeoffs that must be made. In the current stateof the art, design tradeoffs, and design methodologies used have beendirected to integrating all parts of the receiver except for frequenciesselective components. The conventional wisdom in receiver design is thatfilters are not easily integrated onto a silicon substrate and thatfiltering is best done off of a chip.

[0122] Some general design guidelines exist to aid an RF engineer indesigning a receiver. One such rule is that designing for receiverselectivity is more important than designing for receiver sensitivity.Thus, when faced with conflicting design choices, the more desirablechoice is to provide a design that will separate adjacent channels thatinterfere with each other rather than to design a receiver capable ofpicking up the weakest channels. Another rule of thumb in choosingintermediate frequencies is to choose the first intermediate frequencyat twice the highest input frequency anticipated. This is to reduce thepossibility of spurious second order intermodulation distortion.Depending upon a system performance desired, this rule can even be morerestrictive, requiring an IF at greater than three times the highestinput frequency. Thus, it may be seen that a wide variety of performancerequirements exist in a receiver circuit, and that the range of choicesfor a given criteria may be utilized by those skilled in the art toproduce a unique design that meets the challenges posed by an increasingtrend towards integration.

[0123] When more than one IF is present in a receiver there is an imagefrequency associated with each IF that must be considered in the design.A good receiver provides an image rejection greater than 70 dB.

[0124] One of the first considerations in frequency planning asuperheterodyne receiver is the selection of IF conversions. A frequencyrange of the local oscillator needs to be determined to establish thelocations of spurious responses of various orders. Two choices arepossible for each of two possible LO frequency and the selection is notsubject to an easy generalization. The two available frequencies are theabsolute value of the quantity |f_(RF)±f_(IF)|=f_(LO). Selection dependson RF bands chosen to be received and frequencies present in thesebands, the availability of fixed bandwidth filters at a desired IF andconstraints imposed upon an engineer by the limitations of a materialthat will be used to fabricate a receiver.

[0125] Receiver planning is a process that is centered upon frequencyplanning and receiver level diagrams. After initial frequency selectionsfor a frequency plan are made, a receiver level plan is used tocalculate noise figures, intercept points (IP) and levels of spuriousresponses. Each is evaluated in light of design requirements. After eachset of selections performance is evaluated and a next set of parameterselections is made until an appropriate compromise in receiverperformance is achieved.

[0126] Once frequency planning and a level diagram yield a satisfactorydesign solution these tools are used to guide a detailed receiverdesign. Once parameters of a section of a receiver are defined, anengineer can use various circuit implementations to achieve a stateddesign goal. For example a frequency plan and level diagram may requirea band pass filter with certain characteristics such as bandwidth,center frequency and insertion loss. The engineer would then either picka single filter that meets all of these requirements or cascade one ormore filters such that a composite response will yield the requireddesign value.

[0127] Needless to say experience and knowledge of available technologyplays a large part in achieving a successful receiver design blueprint.An engineer must have a rough idea of component availability and designmethodologies that will yield a certain performance. If the engineerspecifies a portion of the receiver that has performance characteristicsthat are not achievable with available components or design methods,then an impractical and unproduceable design has been proposed requiringreplanning the architecture of the receiver.

[0128] A design process and a result achieved is very dependent upontechnology available, materials and methodologies known at the time. Newimprovements in design techniques, computer simulation, processing and apush for increased miniaturization continually fuel achievement of newand innovative receiver designs to solve technological problems.

[0129] Once frequency conversions have been chosen and a receiverdesigned, with the distortion products created in the receiver foundacceptable, the next step in receiver design is to design circuitry thatwill generate one or more local oscillator signals. These signals couldbe provided by a source that is external to a chip. However, this wouldnot be practical in seeking to miniaturize an overall receiver design. Abetter approach is to generate the local oscillator frequencies near thereceiver. In reducing an entire receiver onto a single chip, problems inmaintaining signal purity, and stability are encountered.

[0130] An innovation that has allowed increased miniaturization inreceiver design is the development of frequency synthesis. Localoscillator signals are required in receivers utilizing frequencyconversion. These signals must be tunable and stable. A stable frequencyis easily produced by a quartz crystal at a single frequency. A tunablefrequency can be produced by an LC type oscillator. However, this LCoscillator does not have sufficient stability. Additionally using alarge number of crystals to generate a range of local oscillatorsignals, or inductors required in an LC oscillator do not allow aneasily miniaturized design. Frequency synthesis is space efficient.

[0131] Variable frequency local oscillator signals used in a receivermust be generated by appropriate circuits. These frequency synthesistechniques derive variable LO signals from a common stable referenceoscillator. A crystal oscillator has a stable frequency suitable for usein a synthesizer.

[0132] Oscillators may provide a fixed or a variable output frequency.This fixed or variable frequency may be used for frequency conversion ina receiver as a local oscillator that is used to mix a received radiofrequency (RF) input down to an intermediate frequency or a base bandsignal that is more easily processed in the following circuitry. Anotherway that a received signal can be converted down to a base band orintermediate frequency signal is by using frequency synthesizer outputsas local oscillator signals to mix the signal down. Synthesizers provideaccurate, stable and digitally programmable frequency outputs, withoutthe use of multiple oscillators to tune across a band. Accuracy ismaintained by using feed back.

[0133] Three general techniques are used for frequencies synthesis.Direct synthesizers use frequency multipliers, dividers and mixers.Indirect synthesizers use phase-locked loops. Direct digitalsynthesizers use digital logic combined with a digital to analogconverter to provide an analog output. Some designs combine the threetechniques.

[0134] A direct synthesizer will use a frequency reference such as acrystal oscillator as disclosed in FIG. 5 to generate a referencefrequency. To achieve a desired output frequency, the referencefrequency is multiplied through a series of multipliers. Dividers may beused similarly to reduce the frequency output to the desired lesservalue. Additionally, two signals generated from the chain of multipliersand dividers can be fed into a mixer to generate a third frequency. Themix and divide direct synthesis approach permits the use of manyidentical modules that produce fine resolution with low spurious output.

[0135] Indirect synthesis can take several forms. It can use divide by Nto produce one or more of the digits, and mix and divide with loopsimbedded among circuits. In each form of frequency synthesizer, theloops contained in it are governed by a derivative of a referencefrequency. Indirect synthesis can be used to generate a frequency of$\left( \frac{N}{M} \right){f_{\quad {i\quad n}}.}$

[0136] Circuits of this type are often used as local oscillators fordigitally tuned radio and television receivers.

[0137] Indirect synthesizers make use of a number of phase locked loops(PLLs) in order to create a variety of frequency outputs. Each looppresent in the system makes use of a common frequency reference providedby a single oscillator. Frequency synthesizers provide the advantage ofbeing digitally programmable to a desired frequency as well as providingan extremely stable frequency.

[0138] Frequency stability in a synthesizer is achieved with phaselocked loops. A phase locked loop is programmed to generate a desiredfrequency. Once it approximates the frequency, the frequency is divideddown to the value of a reference frequency, provided by an externaloscillator, and compared to that reference frequency. When thedifference reaches zero the phase locked loop stops tuning and locks tothe frequency that it has just produced. The frequency reference used totune the phase locked loop is typically provided by a single frequencyoscillator circuit.

[0139] Frequency synthesizers in a radio frequency receiver oftenincorporate two phase locked loops. One PLL is used to provide coarsetuning within the frequency band of interest while the second PLLprovides fine tuning steps.

[0140] In using this scheme, a coarse tuning must be such that a desiredchannel will initially fall within the selectivity of the receiver toproduce a signal output. It would be an advantage in receiver design iftuning speed could be increased so that initially several channels wouldfall within the selectivity of the receiver. Tuning in this manner wouldallow an output to be created with an extremely coarse tuning range thatcould be dynamically adjusted. Currently this type of tuning is not seenin the state of the art.

[0141] Typically PLLs use a common reference frequency oscillator. Localoscillator signals produced by a frequency synthesizer's phase lockedloops inject noise produced in the reference frequency oscillator andthe PLLs into a the signal path by way of a PLL output.

[0142] A range of output frequencies from a synthesizer can span manydecades, depending on the design. A “resolution” of the synthesizer isthe smallest step in frequency that can be made. Resolution is usually apower of 10. A “lock up time” of the synthesizer is the time it takes anew frequency to be produced once a command has been made to changefrequencies.

[0143] The more accurate the frequency required the longer the lock uptime. The reduction of the lock up time is a desirable goal insynthesizer design. A modern trend is to use frequency synthesis in wideband tuners. To tune across a wide band width quickly the lock up timemust be minimized. Current state of the art tuning times for jumps infrequencies can be as short as several microseconds. This is difficultto do when the required increment in frequency adjustment is small. Inthe state of the art indirect synthesis is capable of producing multidigit resolution. However, indirect synthesis is not capable ofproviding micro second switching speeds. For faster switching speedsdirect analog and direct digital technologies are used. Therefore, it isdesirable to construct an indirect frequency synthesizer that provideshigh resolution and improved switching speed.

[0144] The present embodiments of the invention allow all channelselectivity and image rejection to be implemented on an integratedcircuit. Integration is a achievable by utilizing differential signaltransmission, a low phase noise oscillator, integrated low Q filters,filter tuning, frequency planning, local oscillator generation and PLLtuning to achieve a previously unrealized level of receiver integration.

[0145] The embodiments of the invention advantageously allow a LCfilters to be integrated on a receiver chip, resulting in an integratedcircuit that contains substantially the entire receiver. Byadvantageously selecting a frequency plan, and utilizing the propertiesof complex mixers, an architecture is achieved that allows LC filters tobe integrated on a receiver chip so that acceptable performance isproduced when converting a received signal to one having a lowerfrequency that is easily processed.

[0146] The embodiments utilize particular aspects of an arbitrarilydefined input spectrum to first shift the received frequencies to ahigher frequency in order that interference may be more easilyeliminated by filtering and then shifting the spectrum to a nominal IFfor processing. This first shifting process advantageously shiftsinterfering image signals away from a center frequency of a first LCfilter bank so that the LC filter bank is more effective in reducing theinterfering signal strength. To further reduce the interfering signalstrength, multiple LC filters that are tuned to the same frequency arecascaded, further reducing the interfering signal strength.

[0147] To reduce degradation of the desired signal the exemplaryembodiments of the invention utilize a complex mixing stage following anLC filter bank to reduce the image frequency interference by anadditional amount that might be necessary to meet a particular imagerejection target (i.e., an about 60 dB to 65 dB rejection target). Acomplex mixer creates a signal as a result of its normal operation thatcancels an image frequency interference by the remaining amount neededto achieve satisfactory performance with LC filters.

[0148] The ultimate goal of a receiver is to reduce the frequency of anincoming signal to a frequency that is lower than received, so thatprocessing of the desired signal can be easily achieved. The receiverarchitecture utilizes two frequency down conversions to achieve thisgoal. Each frequency conversion is susceptible to interference thatrequires filtering. Frequency planning as described above used inconjunction with LC filters and complex mixers, provides the requiredimage distortion rejection that allows LC filters to be usedadvantageously in an integrated receiver.

[0149] Radio receivers require one or more local oscillator (LO) signalsin order to accomplish frequency conversion to an intermediate (IF)frequency. In a typical receiver these local oscillator signals must bestable and free from noise. When a receiver is fabricated as anintegrated circuit, the chances of injecting noise via the LO signalsincreases. Local oscillator signals for a receiver are typicallygenerated in close proximity to the frequency conversion circuitry. Theclose proximity of this frequency generation circuitry to the signalpath creates an increased likelihood of noise being radiated orconducted to cause interference with the received signal.

[0150] In order to achieve improved noise immunity the exemplaryembodiments of the invention may utilize circuitry to generate the localoscillator signals that possess superior noise performance. The localoscillator signals may also be advantageously transmitted differentiallyto the mixers present on the integrated circuit. It should be noted thatin alternate embodiments of the invention that a single ended output canbe produced from the differential signal by various techniques known inthe art. This technique is used advantageously whenever externalconnections to the receiver are required that are single ended.

Oscillator

[0151] An exemplary embodiment of the present invention utilizes adifferential oscillator having low phase noise or jitter and highisolation, as a frequency reference that substantially increases theperformance of a tuner architecture integrated onto a single siliconsubstrate.

[0152] In accordance with the present invention, a crystal oscillatorcircuit is provided and constructed so as to define a periodic,sinusoidal, balanced differential signal across two symmetricalterminals of a crystal resonator which are coupled in a parallelconfiguration across symmetrical, differential terminals of adifferential oscillator circuit.

[0153] The differential oscillator circuit is configured such that it isconstructed of simple active and passive components which are easilyimplemented in modern integrated circuit technology, thus allowing thedifferential oscillator circuit to be accommodated on a monolithicintegrated circuit chip for which the crystal oscillator (as a whole) isproviding a suitable, stable periodic timing reference signal.Similarly, and in contrast to prior art implementations, only theresonating crystal (crystal resonator or quartz crystal resonator) isprovided as an off-chip component. This particular configuration allowsfor considerable savings in component parts costs by partitioning moreand more functionality into the integrated circuit chip.

[0154] Remote (off chip) mounting of the crystal resonator requires thatelectrical contact between the crystal resonator and the associatedoscillator circuit, be made with interconnecting leads of finite length.In integrated circuit technology, these interconnecting leads aretypically implemented as circuit pads and conductive wires formed on aPC board substrate to which package leads are bonded (soldered) in orderto effect electrical connection between the crystal resonator and anassociated oscillator circuit. External electrical connections of thistype are well known as being susceptible to noise and other forms ofinterference that might be radiated onto the interconnecting leads and,thence, into the oscillator circuit, degrading its overall noiseperformance.

[0155] A sinusoidal signal source, having a differential outputconfiguration, defines a pair of periodic sinusoidal signals, with thesignal at one output terminal defined as being 180° out of phase with asimilar periodic, sinusoidal signal appearing at the other outputterminal. Classical differential signals are termed “balanced” in thatboth signals exhibit equal peek-to-peek amplitudes although they exhibita 180° phase relationship. As illustrated in the simplified timingdiagram of FIG. 6, differential signals have a particular advantage inthat common-mode interference, that is injected on either terminal, iscanceled when the signal is converted to single-ended. Such common modeinterference is typically of equal amplitude on each pin and is causedby radiation into the circuit from external sources or is oftengenerated in the circuit itself. In FIG. 6, a positive sinusoidalsignal, denoted signal-P oscillates about a zero reference, but isshifted by a common-mode interference component, denoted ICM. Likewise,a negative sinusoidal signal, denoted at signal-n, also oscillates abouta zero reference, exhibiting a 180° phase relationship with signal-p,and is also offset by a common mode interference component denotedI_(CM).

[0156] A superposition of the positive and negative periodic signals isillustrated in the timing diagram denoted “composite”, which clearlyillustrates that the peek-to-peek difference between the positive andnegative signals remains the same, even in the presence of a common modeinterference component ICM.

[0157] Turning now to FIG. 7, there is depicted a semi-schematic blockdiagram of a periodic signal generation circuit including a differentialcrystal oscillator driving a differential linear buffer amplifier.Advantageously, the present invention contemplates differential signaltransmission throughout its architecture to maintain the purity of thederived periodic signal and to minimize any common mode interferencecomponents injected into the system. In particular, the presentinvention incorporates differential signal transmission in theconstruction of a differential crystal oscillator circuit, including acrystal resonator and its associated oscillator driver circuit.Differential signal transmission is maintained through at least a firstlinear buffer stage which functions to isolate the differentialoscillator circuit switch transients and other forms of noise that mightbe generated by follow-on digital integrated circuit components.

[0158] In FIG. 7, a differential crystal oscillator circuit isconfigured to function as a source of stable, synchronous and periodicsignals. According to the illustrated embodiment, a differential crystaloscillator 710 suitably incorporates a resonating crystal 712 and a pairof symmetrical load capacitors 714 and 716, each load capacitorrespectively coupled between ground potential and one of the twosymmetrical output terminals of the resonating crystal 712.

[0159] Resonating crystal 712 is coupled between differential terminalsof a differential oscillator driver circuit 718, in turn connected todifferential inputs of a differential linear buffer integrated circuit720. The symmetrical terminals of the resonating crystal 712 are coupledacross differential terminals of the resonator and linear buffer, with afirst terminal of the crystal being shunted to ground by the first shuntcapacitor 14. The second terminal of the crystal is shunted to ground bythe second shunt capacitor 716.

[0160] The oscillator driver circuit portion of the differential crystaloscillator 710 functions, in cooperation with the crystal resonator 712,to define a pure sinusoidal and differential signal across the crystal'ssymmetrical terminals. As will be developed in greater detail below,this pure sinusoidal and differential signal is then used by the linearbuffer 720 to develop an amplified representation of periodic signalssynchronized to the crystal resonant frequency. These amplified signalsare also contemplated as differential inform and are eminently suitablefor driving digital wave shaping circuitry to define various digitalpulse trains useable by various forms of digital timing circuitry, suchas phase-lock-loops (PLLs), frequency tunable digital filters, directdigital frequency synthesizers (DDFS), and the like. In other words, thesystem depicted in FIG. 7 might be aptly described as a periodicfunction generator circuit, with the crystal oscillator portion 710providing the periodicity, and with the buffer portion 720 providing thefunctionality.

[0161] Before entering into a detailed discussion of the constructionand operation of the differential oscillator driver circuit anddifferential linear buffer amplifier, it will be useful to describecharacteristics of a resonating crystal, such as might be contemplatedfor use in the context of the present invention.

[0162]FIG. 8 depicts the conventional representation of a resonatingcrystal 712 having mirror-image and symmetrical terminals 822 and 824,upon which differential periodic signals may be developed at thecrystal's resonant frequency. Resonating crystals (also termed crystalresonators) may be formed from a variety of resonating materials, butmost commonly are formed from a piece of quartz, precisely cut alongcertain of its crystalline plane surfaces, and so sized and shaped as todefine a particular resonant frequency from the finished piece.Resonating crystals so formed are commonly termed “quartz crystalresonators”.

[0163] A typical representational model of the equivalent circuit of aquartz crystal resonator 712 is illustrated in simplified,semi-schematic form in FIG. 9. A quartz crystal resonator can be modeledas a two terminal resonator, with an LCR circuit, incorporating acapacitor C_(m) in series with an inductor L_(m) and a resistor R_(m),coupled in parallel fashion with a capacitor C₀ across the twoterminals. It will be understood that the particular component values ofthe capacitor, inductor and resistor, forming the LCR filter portion ofthe circuit, define the resonant characteristics of the crystal. Thesedesign values may be easily adjusted by one having skill in the art inorder to implement a resonating crystal operating at any reasonablydesired frequency.

[0164] For example, a particular exemplary embodiment of a crystalresonator might be desired to have a resonant frequency in the range ofabout 10 megahertz (MHz). In such a case, the equivalent circuit of sucha crystal might have a typical value of about 20 femto Farads (fF) forthe capacitor C_(m). The inductor L_(m) might exhibit a typical value ofabout 13 milli Henreys (mH), while the resistor might have a typicalvalue of about 50 ohms. When used in a practical oscillator design,oscillation will be achieved for values of the capacitor C₀ that areless than a design worst case value. In the exemplary embodiment, worstcase values of 7 pico Farads (pF) might be chosen in order to ensure adesign that oscillates at the desired resonant frequency over a widerange of crystal equivalent circuit values. In a practical application,the typical range of capacitance values for C₀ might be from about 3 toabout 4 pF.

[0165]FIGS. 10 and 11 are graphical representations depicting responseplots of impedance and phase with respect to frequency, respectively, ofa crystal resonator circuit constructed in accordance with theequivalent circuit model of FIG. 9 and using the values given above forthe component C_(m), L_(m), R_(m), and C₀ parts. FIG. 10 is a plot ofthe real portion of impedance, in ohms, as a function of the resonator'sfrequency and mega Hertz. FIG. 11 is a representational plot of theimaginary impedance component (expressed as phase), again expressed as afunction of frequency in mega Hertz. From the representational plots, itcan be understood that an exemplary crystal resonator constructed inaccordance with the above values exhibits a resonant frequency in therange of about 10 MHz. Further, simulation results on such a crystalresonator exhibit a steep rise in the real impedance versus frequencyplot of FIG. 10 in the resonance region about 10 MHz. A steep rise inreal impedance in the resonance region is indicative of a high qualityfactor, Q, typically exhibited by quartz crystal resonators.

[0166] An example of a quartz crystal resonator having theaforementioned characteristics and exhibiting a resonance fundamental atabout 10 MHz is a Fox HC49U, quartz crystal resonator, manufactured andsold by Fox Electronics of Ft. Myers, Fla. It should be noted, however,that the specific values of a quartz crystal resonator, including itsresonant frequency, are not particularly important to practice ofprinciples of the invention. Any type of crystal resonator may be usedas the resonator component 712 of FIG. 7, so long as it is constructedwith generally symmetrical terminals which can be driven, in a manner tobe described in greater detail below, by an oscillator driver circuit718 of FIG. 7 so as to develop a differential, sinusoidal signal withrespect to the two terminals. Further, the resonator need not oscillateat a frequency of 10 MHz. The choice of resonant frequency is solely afunction of a circuit designer's preference and necessarily depends onthe frequency plan of an integrated circuit in which the system of theinvention is used to provide periodic timing signals.

[0167] Turning now to FIG. 12, there is depicted a simplified schematicdiagram of a differential oscillator driver circuit, indicated generallyat 718, suitable for differential coupling to a crystal resonator inorder to develop balanced, differential sinusoidal signals for use bydownstream components.

[0168] In the exemplary embodiment of FIG. 12, the differentialoscillator driver circuit 718 is constructed using common integratedcircuit components and is symmetrical about a central axis. Theoscillator driver 718 is constructed with a pair of P-channeltransistors 1226 and 1228 having their source terminals coupled incommon and to a current source 1230 connected, in turn, between thecommon source terminals and a positive supply potential V_(DD). The gateterminals of each of the P-channel transistors 1226 and 1228 are coupledto the drain nodes of the opposite transistor, i.e., the gate terminalof P-channel transistor 1228 is coupled to the drain node of P-channeltransistor 1226, and vice versa.

[0169] Output terminals are defined at each of the transistor's drainnodes, with the drain node of P-channel transistor 1226 defining the“negative” terminal (Von) and the drain terminal of P-channel transistor1228 defining the “positive” output (Vop). Thus, it will be understoodthat the circuit is able to operate differentially by cross coupling thetransistors 1226 and 1228 in order to provide feedback.

[0170] Because transistors exhibit some measure of gain at allfrequencies, particularly DC, conventional cross coupled transistors areoften implemented as latches in digital circuit applications where largeDC components are present. In the differential oscillator driver circuit718 of the invention, latching is prevented by removing the DC gaincomponent, while retaining the system's high frequency gain,particularly gain in the desirable 10 MHz region.

[0171] In order to substantially eliminate the gain component at lowfrequencies, a high pass filter is interposed between the gate andoutput terminals of each symmetrical half of the circuit. In particular,a high pass filter 1232 is coupled between the “negative” outputterminal and the gate terminal of P-channel transistor 1228. Likewise,the high pass filter 1234 is coupled between the “positive” outputterminal and the gate terminal of P-channel transistor 1226. Further,each of the high pass filters 1232 and 1234 are coupled between avirtual ground, identified as Vmid and indicated in phantom in theexemplary embodiment of FIG. 12, and the corresponding gate terminal ofthe respective one of the differential pair P-channel transistors 1226and 1228. Each of the high pass filters 1232 and 1234 are implemented asRC filters, each including a resistor and capacitor in a series-parallelconfiguration. Each capacitor is series-connected between an outputterminal and the gate terminal of a corresponding differential pairtransistor, while each resistor is coupled between a gate terminal andthe virtual ground. Thus, the first high pass filter 1232 includes acapacitor 1236 coupled between the “negative” terminal and the gateterminal of P-channel transistor 1228. A resistor 1238 is coupledbetween the gate of P-channel transistor 1228 and virtual ground.Similarly, the second high pass filter 1234 includes a capacitor 1240coupled between the “positive” terminal and the gate terminal ofP-channel transistor 1226. A resistor 1242 is coupled between the gateof P-channel transistor 1226 and the virtual ground.

[0172] In operation, high pass filter 1232 filters the input from Vonprior to applying that signal to the gate of its respective differentialpair transistor 1228. In like manner, high pass filter 1234 filters theinput from Vop prior to applying that signal to the gate of itsrespective differential pair transistor 1226. Each of the high passfilters are symmetrically designed and have component values chosen togive cutoff frequencies in the range of about 5 MHz. For example, filtercapacitors 1236 and 1240 might have values of about 1.5 pF, and filterresistors 1238 and 1242 might have values in the range of about 718Kohms. Which would give a filter yielding the desired 5 MHz cutoff. Itwill be thus understood that the differential oscillator driver circuit18 will have negligible gain at DC, while exhibiting its design gainvalues in the desired region of about 10 MHz.

[0173] It should be understood that the component values for high passfilters 1232 and 1234 were chosen to give a particular cut off frequencyof about 5 MHz, allowing the oscillator driver circuit to exhibit fulldesign gain at a resonate frequency of about 10 MHz. If the resonantfrequency of the crystal oscillator circuit were required to have adifferent value, the components of the high pass filters 1232 and 1234would necessarily take on different values to accommodate the differentoperational characteristics of the circuit. Accordingly, the actualcomponent values, as well as the cutoff frequency value of the exemplaryembodiment, should not be taken as limiting the differential oscillatordriver circuit according to the invention in any way. The values andcharacteristics of the differential oscillator driver circuit 18 of FIG.12 are exemplary and have been chosen to illustrate only one particularapplication.

[0174] Because the common mode output signal of a differential amplifieris often undefined, the differential oscillator driver circuit 718 ofFIG. 12 is provided with a common mode control circuit which functionsto maintain any common mode output signal at reasonable levels. Inparticular, a differential pair of N-channel transistors 1244 and 1246is provided with each having its drain terminal coupled to a respectiveone of the Von and Vop output terminals. The differential N-channeltransistors 1244 and 1246 further have their source terminals tiedtogether in common and to a negative supply potential V_(SS). Their gateterminals are tied together in common and are further coupled, infeedback fashion, to each transistor's drain node through a respectivebias resistor 1248 and 1250. The bias resistors 1248 and 1250 each havea value, in the exemplary embodiment, of about 100 Kohms, with the gateterminals of the N-channel differential pair 1244 and 1246 coupled to acenter tab between the resistors. This center tab defines the virtualground Vmid which corresponds to a signal midpoint about which thesinusoidal signals Von and Vop oscillate. Any common mode componentpresent at the outputs will cause a voltage excursion to appear at thegates of the N-channel differential pair 1244 and 1246. In other words,virtual ground Vmid can be thought of as an operational threshold forthe current mode control differential pair 1244 and 1246. Common modeexcursions above or below Vmid will cause a common mode controldifferential pair to adjust the circuit's operational characteristics soas to maintain Vmid at a virtual ground level, thus minimizing anycommon mode component.

[0175] In operation, noise in such a linear differential oscillatordriver circuit is filtered mainly by the crystal resonator, but also bythe operational characteristics of the driver circuit. For example,noise at 10 MHz is amplified by the positive feedback characteristics ofthe circuit and will continue to grow unless it is limited. In theexemplary embodiment of FIG. 12, signals in the 10 MHz region willcontinue to grow in amplitude until limited by a non-linearself-limiting gain compression mechanism.

[0176] As the amplitude of the amplified signal becomes large, theeffective transconductance g_(m) of the P-channel differential pairtransistors 1226 and 1228 fall off, thus limiting the gain of thedifferential amplifier. Amplifier gain falloff with increasing gatevoltage excursions is a well understood principle, and need not bedescribed in any further detail herein. However, it should be mentionedthat as the gain of the oscillator driver circuit trends to 1 thecrystal resonator begins to self-limit, thus defining a constant outputamplitude sinusoidal signal. Constancy of the amplitude excursions arereflected to the control (gate) terminals of the P-channel differentialpair 1226 and 1228 where the feedback mechanism ensures stability aboutunity gain.

[0177] It should be understood therefore that the differentialoscillator driver circuit 718 in combination with a crystal resonator(712 of FIG. 7) function to define periodic, sinusoidal and differentialsignals across the terminals of the crystal resonator. The signals aredifferential in that they maintain a 180° phase relationship. Signalquality is promoted because the exemplary differential oscillator drivercircuit is designed to be highly linear with a relatively low gain, thusreducing phase noise (phase jitter) to a significantly better degreethan has been achieved in the prior art. Signal quality and symmetry isfurther enhanced by the symmetrical nature of the two halves of theoscillator driver circuit. Specifically, the oscillator driver circuitis symmetrical about a central axis and, when implemented in integratedcircuit technology, that symmetry is maintained during design andlayout. Thus, conductive signal paths and the spatial orientation of thedriver's active and passive components are identical with respect to the“negative” and “positive” outputs, thereby enhancing signal symmetry andfurther minimizing phase jitter.

[0178] In accordance with the invention, differential crystal oscillatorcircuit is able to provide a periodic clock signal (approximately 10MHz) that exhibits stable and robust timing characteristics with verylow jitter. As depicted in the simplified semi-schematic block diagramof FIG. 13, a particular exemplary embodiment of a periodic signalgenerator circuit incorporates a differential crystal oscillator circuitaccording to the present invention, including a crystal resonator 12 anddifferential oscillator driver circuit 718. A resonant crystal circuit12 includes first and second timing capacitors (714 and 716 of FIG. 7)which are not shown merely for convenience in ease of explanation. Theresonant crystal circuit 712 is coupled, in parallel fashion, across theoutput terminals of the oscillator driver circuit 718 which incorporatesthe active device circuitry for pumping energy into the circuit in orderto sustain oscillation. This parallel combination is coupled,differentially, into a linear buffer amplifier 720, which functions toprovide a linear gain factor K to the differential signal provided bythe crystal oscillator circuit.

[0179] Linear buffer amplifier 720 provides signal isolation, throughhigh input impedance, as well as amplification of the oscillating (10MHz) signal produced by the crystal resonator/oscillator drivercombination. Linear buffer amplifier 720 is configured to outputdifferential mode signals characterized by linear amplification of theinput differential signals, that may then be used to drive one or moreadditional wave shaping-type devices, such as nonlinear bufferamplifiers 1352, 1354 and 1356.

[0180] In the exemplary embodiment of FIG. 13, the nonlinear buffers1352, 1354 and 1356 function in order to provide signal translation(wave shaping) from the differential sign wave periodic signal presentat the output of the linear buffer 720 to a digital pulse train atcharacteristic logic levels suitable for driving fall-on digital circuitblocks 1358, 1360 and 1362. In addition to its signal translationfunction, nonlinear buffers 1352, 1354 and 1356 also provide a measureof signal conditioning, transforming the purely sinusoidal signal attheir inputs to a very low jittergetter square wave output.

[0181] Following digital circuitry 1358, 1360 and 1362 illustrated inthe exemplary embodiment of FIG. 13 might be any type of digitalcircuitry that requires a stable periodic clock, such as aphase-lock-loop, a tunable filter, a digital frequency synthesizer, andthe like. Characteristically, high speed switching circuits of thesetypes generate a great deal of noise, particularly as a result of groundbounce, switch transients and ringing. In order to minimize feed throughcoupling of these noise sources back to the crystal oscillator circuit,and in contrast to the prior art, the system of the present inventionutilizes two stages of buffering.

[0182] In the prior art, signal transformation from a sinusoidal signalto a square wave output is typically implemented by using an inverter tosquare sinusoidal input signal. A digital inverter function might becharacterized as a nonlinear amplifier of a transformed sinusoidal inputsignal to a square wave by providing an extremely high gain, such thatthe input signal is driven to the rail during amplification (i.e.,clipping). Thus, the output signal of a typical inverter might becharacterized as a clipped sine wave. This particular nonlinearitycharacteristic of the inverter further provides opportunities for phasenoise to be added to the output signal.

[0183] Phase noise (phase jitter) can also be introduced when the slopeof a signal waveform going through a zero transition is not sharp. Thus,in the present invention, phase noise is minimized in the nonlinearbuffer amplifiers 1352, 1354 and 1356 by amplifying the differentialsignal provided by the crystal oscillator circuit through the linearamplifier 720 in order to increase the amplitude, and thus the slewrate, of the signal prior to its conversion to a square wave. Phasenoise resulting from zero crossings of the nonlinear buffer amplifiersis thereby minimized.

[0184] Further, in a very large scale integrated circuit, there are agreat number of digital logic elements coupled to a common power supply.Switching of these digital logic elements causes the power supplyvoltage to move up and down, causing digital switching noise. Thismovement in the power supply induces a jitter component at each inverterthat is used as a buffer in a conventional oscillator circuit. Accordingto the present invention, maintaining a differential signal throughoutthe oscillator circuit, including the wave shaping buffers, allows theeffects of power supply noise to be substantially eliminated from theoscillator, thus maintaining signal quality. In addition, the use of adifferential signal throughout the oscillator's architecture allowscommon mode noise radiated onto the pins of the crystal resonator to berejected.

[0185] The number of nonlinear buffers which might be cascaded in orderto produce a suitable clock signal is an additional important feature inthe design of a low phase noise oscillator circuit. In conventionaloscillator circuits, multiple cascaded invertors are used to providehigh isolation of the final, squared output signal. In such cases, eachtime the signal passes through a nonlinear inverter, zero crossingoccurs which offers an additional opportunity for phase noise to beadded to the circuit. In order to minimize phase noise, the presentinvention contemplates a single stage of nonlinear buffering whichpresents a high input impedance to the linear buffer 720 which proceedsit. Additionally, the linear buffer 720 is further provided with a highinput impedance to further isolate the crystal resonator and itsassociated differential oscillator driver circuitry from noise loading.

[0186] An exemplary embodiment of a linear buffer suitable for use inconnection with the periodic signal generation circuit of FIG. 13 isillustrated in simplified, semi-schematic form in FIG. 14. The exemplaryembodiment of FIG. 14 illustrates the conceptual implementation of adifferential-in differential-out amplifier. The differentialimplementation has several advantages when considered in practicalapplications. In particular, maximum signal swing is improved by afactor of 2 because of the differential configuration. Additionally,because the signal path is balanced, signals injected due to powersupply variation and switch transient noise are greatly reduced.

[0187] The exemplary implementation of a differential-in,differential-out amplifier (indicated generally at 720) of FIG. 14 usesa folded cascade configuration to produce a differential output signal,denoted V_(out). Since the common-mode output signal of amplifiershaving a differential output can often be indeterminate, and thus causethe amplifier to drift from the region where high gain is achieved, itis desirable to provide some form of common-mode feedback in order tostabilize the common-mode output signal. In the embodiment of FIG. 14,the common-mode output signal is sampled, at each of the terminalscomprising the output V_(out) and fed back to the current-sink loads ofthe folded cascade.

[0188] Differential input signals V_(in) are provided to the controlterminals of a differential input pair 1464 and 1466, themselves coupledbetween respective current sources 1468 and 1470 and to a commoncurrent-sink load 1472 to V_(SS). Two additional transistors (P-channeltransistors in the exemplary embodiment of FIG. 14) define the cascadeelements for current-sources 1468 and 1470 and provide bias current tothe amplifier circuit.

[0189] High impedance current-sink loads at the output of the amplifier1476 and 1478 might be implemented by cascoded current sink transistors(N-channel transistors for example) resulting in an output impedance inthe region of about -b 1 Mohm. The common mode feedback circuit 1480might be implemented as an N-channel differential pair, biased in theiractive regions and which sample the common-mode output signal andfeedback a correcting, common-mode signal into the source terminals ofthe cascoded transistors forming the current-sinks 1476 and 1478. Thecascade devices amplify this compensating signal in order to restore thecommon-mode output voltage to its original level.

[0190] It should be noted that the exemplary linear amplifier of FIG. 14might be implemented as any one of a number of appropriate alternativeamplifiers. For example, it need not be implemented as a fullydifferential folded cascade amplifier, but might rather be implementedas a differential-in, differential-out op amp using two differential-insingle-ended out op amps. Further, the actual circuit implementationmight certainly vary depending on the particular choices and prejudicesof an analog integrated circuit designer. The input differential pairmight be either an N-channel or a P-channel pair, MOS devices might beused differentially as active resistors or alternatively, passiveresistor components might be provided, and the like. All that isrequired is that the linear amplifier 720 amplifies a differential inputsignal to produce a differential, sinusoidal signal at its output. Thus,the only frequency components reflected back through the linearamplifier 720 will be sinusoidal in nature and thus, will not affect theoperational parameters of the differential crystal oscillator frequency.Further, the linear buffer 720 will necessarily have a relatively highoutput impedance in order to attenuate noise that might be reflectedback from the square wave output of the following nonlinear amplifierstages.

[0191] Turning now to FIG. 15, there is depicted a simplifiedsemi-schematic diagram of a nonlinear buffer, indicated generally at1582, such as might be implemented as a wave shaping or squaring circuit1352, 1354 or 1356 of FIG. 13. The nonlinear buffer 1582 receives adifferential, sinusoidal input signal at the gate terminals of an inputdifferential transistor pair 1584 and 1586. Drain terminals of thedifferential pair 1584 and 1586 are connected together in common and toa current sink supply 1588 which is coupled to a negative potential.Each of the differential pairs' respective source terminals are coupledto a bias network, including a pair of differential bias transistors1590 and 1592 having their gate terminals tied together in common andcoupled to a parallel connected bias network. The bias network issuitably constructed of a resistor 1594 and a current sink 1596connected in series between a positive voltage potential such as Vdd andVss. A bias node between the resistor 1594 and current sink 1596 iscoupled to the common gate terminals of the bias transistor network 1590and 1592 and defines a bias voltage for the bias network which will beunderstood to be the positive supply value minus the IR drop across biasresistor 1594. The current promoting the IR drop across the biasresistor 1594 is, necessarily, the current I developed by the currentsink 1596.

[0192] A differential, square wave-type output (Vout) is developed attwo output nodes disposed between the respective source terminals of thebias network transistors 1590 and 1592 and a respective pair of pull-upresistors 1598 and 1599 coupled, in turn, to the positive supplypotential. It should be noted, that the bias network, includingtransistors 1590 and 1592, function to control the non-linearamplifier's common mode response in a manner similar to the linearamplifier's common mode network (transistors 1244 and 1246 and resistors1248 and 1250 of FIG. 12).

[0193] Although depicted and constructed so as to generate adifferential square wave-type output in response to a differentialsinusoidal input signal, the non-linear buffer 1582 of FIG. 15 is wellsuited for single-ended applications as well as for differentialapplications. If a single-ended output is desired, one need only take asignal from one of the two symmetric outputs. The choice of whether toimplement the non-linear buffer as a single-ended or a differentialbuffer will depend solely on the input requirements of any follow-ondigital circuitry which the periodic signal generation circuit inaccordance with the invention is intended to clock. This option issolely at the discretion of the system designer and has no particularbearing on practice of principles of the invention.

[0194]FIG. 16 is a semi-schematic illustration of an alternativeembodiment of the differential oscillator driver circuit (718 of FIG.12). From the exemplary embodiment of FIG. 16, it can be understood thatthe oscillator driver circuit is constructed in a manner substantiallysimilar to the exemplary embodiment of FIG. 12, except that a crystalresonator is coupled across the circuit halves above the differentialtransistor pair, as opposed to being coupled across a circuit from theVon to Vop output terminals. The alternative configuration of FIG. 16operates in substantially the same manner as the embodiment of FIG. 12and produces the same benefits as the earlier disclosed oscillator. Itis offered here as an alternative embodiment only for purposes ofcompleteness and to illustrate that the specific arrangement of theembodiment of FIG. 12 need not be followed with slavish precision.

[0195] It should be understood that oscillator circuits with low phasenoise are highly desirable in many particular applications. FIG. 17illustrates one such application as a reference signal generator in aphase-lock-loop. The phase-lock-loop uses a low phase noise periodicsignal generation circuit in accordance with the invention in order togenerate a reference signal for use by a phase detector. Providing aclean reference signal to the phase detector is fundamental to providinga clean RF output from the PLL. Since noise and nonlinearities inducedby signal generation circuit are carried through the PLL circuit, thusdegrading the RF output, reducing phase noise and providing noiserejection early on in the signal processing chain is advantageous tomaintaining a clean RF output. A differential crystal oscillator (710 ofFIG. 7) advantageously provides this claim signal by maintaining adifferential signal across the terminals of the resonating crystal, animprovement not currently available in state-of-the-art crystaloscillators. Additionally, the use of linear buffer amplifiers followedby nonlinear amplification in a reference oscillator circuit is a uniqueimprovement over the prior art in reducing phase noise.

[0196] Since PLLs have become available in integrated circuit form, theyhave been found to be useful in many applications. Certain examples ofadvantageous application of phase-lock-loop technology include trackingfilters, FSK decoders, FM stereo decoders, FM demodulators, frequencysynthesizers and frequency multipliers and dividers. PLLs are usedextensively for the generation of local oscillator frequencies in TV andradio tuners. The attractiveness of the PLL lies in the fact that it maybe used to generate signals which are phase-locked to a crystalreference and which exhibit the same stability as the crystal reference.In addition, a PLL is able to act as a narrow band filter, i.e.,tracking a signal whose frequency may be varying.

[0197] A PLL uses a frequency reference source in the control loop inorder to control the frequency and phase of a voltage control oscillator(VCO) in the loop. The VCO frequency may be the same as the referencefrequency or may be a multiple of the reference frequency. With aprogrammable divider inserted into the loop, a VCO is able to generate amultiple of the input frequency with a precise phase relationshipbetween a reference frequency and an RF output. In order to maintainsuch a precise phase and frequency relationship, the frequency referenceprovided to the PLL must, necessarily, also be precise and stable.

[0198]FIG. 18 is a simplified block diagram of an illustrative frequencysynthesizer that might incorporate the differential periodic signalgeneration circuit of the invention. The frequency synthesizer is asignal generator that can be switched to output any one of a discreteset of frequencies and whose frequency stability is derived from acrystal oscillator circuit.

[0199] Frequency synthesizers might be chosen over other forms offrequency sources when the design goal is to produce a pure frequencythat is relatively free of spurious outputs. Particular design goals infrequency synthesizer design might include suppression of unwantedfrequencies and the suppression of noise in a region close to theresonant frequency of the crystal that is a typical source of unwantedphase modulation. Synonymous terms for this type of noise are broadbandphase noise, spectral density distribution of phase noise, residual FM,and short term fractional frequency deviation.

[0200] To reduce the noise produced in a synthesizer, crystaloscillators are commonly used due to their stability and low noiseoutput. The use of a periodic signal generation circuit incorporating adifferential crystal oscillator according to an embodiment of thepresent invention advantageously improves these performance parameters.Improved phase noise is achieved through the use of linear bufferingfollowed by nonlinear amplification, while noise rejection is providedby the differential design utilized throughout the circuitryarchitecture.

[0201] It should be evident that a periodic signal generation circuitaccording to the invention has many uses in modern, state-of-the-arttiming circuits and systems. The periodic signal generation circuit isconstructed of simple active and passive components which are easilyimplemented in modern integrated circuit technology. Thus allowingsubstantially all of the components to be accommodated on one monolithicintegrated circuit chip for which the crystal oscillator portion isproviding a suitable, stable periodic timing reference signal. Only theresonating crystal portion (crystal resonator or quartz crystalresonator) is provided as an off-chip component. This particularconfiguration allows for considerable savings in component parts costsby partitioning more and more functionality into the integrated circuitchip itself.

[0202] A more detailed description of the oscillator is provided in U.S.patent application Ser. No. ______ filed ______ (B600:33758) entitled“Differential Crystal Oscillator” by Christopher M. Ward and PieterVorenkamp; based on U.S. Provisional Application No. 60/108,209 filedNov. 12, 1998 (B600:33588), the subject matter of which is incorporatedin its entirety by reference. The oscillator's output is a differentialsignal that exhibits high common mode noise rejection. Use of a lownoise reference oscillator with differential signal transmission allowsthe synthesis of stable low noise local oscillator signals.Advantageously in the present exemplary embodiment of the invention aunique generation of the local oscillator signals allows completeintegration of a receiver circuit on a CMOS integrated circuit byreducing noise in the signal path.

[0203] Frequency synthesizers and a radio frequency receiver oftenincorporate phase locked loops that make use of a crystal oscillator asa frequency reference. A PLL is used to provide coarse tuning within thefrequency band of interest while a second PLL provides fine tuningsteps. Advantageously, the present embodiments of the invention utilizea method of coarse/fine PLL adjustment to improve the performance of theintegrated tuner.

Coarce/fine PLL Adjustment

[0204]FIG. 19 is a diagram illustrating receiver tuning. The combinationof a wide band PLL 1908 and a narrow band PLL 1910 tuning provides acapability to fine tune a receiver's LOS 1902, 1904 over a largebandwidth in small frequency steps. For the exemplary embodiments of QAMmodulation a small frequency step is 100 kHz, and 25 kHz for NTSCmodulation. Fine tuning is available over an entire exemplary 50 MHz to860 MHz impact frequency band width 1906. The first PLL 1908 tunes afirst LO 1902 in large 10 MHz frequency steps and the second PLL 1910tunes a second LO 1904 in much smaller steps. The first intermediatefrequency (IF) filter 1912 has a sufficiently wide band width to allowup to 10 MHz frequency error in tuning the first intermediate frequency,with the narrow band PLL providing final fine frequency tuning toachieve the desired final IF frequency 1914.

[0205]FIG. 20 is a block diagram of an exemplary tuner 2002 designed toreceive a 50 to 860 MHz bandwidth signal 2004 containing a multiplicityof channels. In this exemplary band of frequencies, there are 136channels with a spacing between channel center frequencies of sixmegahertz 2008. The tuner selects one of these 136 channels 2006 thatare at a frequency between 50 and 860 MHz by tuning to the centerfrequency of the selected channel 2010. Once a channel is selected thereceiver rejects the other channels and distortion presented to it. Theselected channel is down converted to produce a channel centered about a44 MHz intermediate frequency (IF) 2012. Alternatively the value of theintermediate frequency ultimately produced by the tuner may be selectedutilizing the method of the invention to provide any suitable final IFfrequency, such as 36 MHz In selecting one of these 136 channels, amaximum frequency error in the local oscillator (LO) frequency used totune the channel to a given IF of plus or minus 50 kHz is allowable.Using one frequency conversion to directly tune any one of the 136channels to 44 MHz would require a tuning range in the local oscillatorof 810 MHz. This would require a local oscillator that tunes from 94 to854 MHz, if utilizing high side conversion.

[0206] Achieving this with a single LO is impractical. Tuning range inlocal oscillators is provided by varactor diodes that typically require33 volts to tune them across their tuning range. Additionally, withinthis tuning range a frequency tuning step of 100 kHz is required toensure that the center frequency of a tuned channel is tuned within plusor minus 50 kHz. Thus, a large range of frequencies would have to betuned in small increments over a 33 volt tuning signal range.

[0207] Returning to FIG. 19 illustrating the frequency tuning method ofthe invention an exemplary 50 to 860 MHz signal 1906 is presented to afirst mixer 1916 that is tuned with a wide band PLL 1908 that tunes afirst LO 1902 in frequency steps of 10 MHz. This local oscillator 1902is set to a frequency that will nominally center a channels that hasbeen selected at a first IF of 1,200 MHz 1918. The first IF 1918 is thenmixed 1920 to the second IF of 275 MHz 1922. This is done by the narrowband PLL 1910 that tunes a second LO 1904 in frequency steps of 100 kHz.The second IF 1922 is next mixed 1924 down to a third IF 1926 of 44 MHzby a third local oscillator signal 1928. This third local oscillatorsignal 1930 is derived from the second local oscillator or narrow bandPLL signal by dividing its frequency by a factor of four.

[0208]FIG. 21 is an exemplary table of frequencies utilizing coarse andfine PLL tuning to derive a 44 MHz IF (“IF-3”). A process is utilized todetermine the wide and narrow band PLL frequencies. The relationshipbetween the wideband PLL and narrowband PLL frequencies to yield thedesired intermediate frequency is found from:

FLO 1−Fsig−(5/4*FLO 2)=Fif  (4)

[0209] where:

[0210] FLO1: PLL1 frequency (10 MHz steps)

[0211] FLO2: PLL2 frequency (e.g., 25 kHz/100 kHz/200 kHz or 400 kHzstep)

[0212] Fsig: Input signal

[0213] Fif (e.g., 44 MHz or 36 MHz or whatever IF is required)

[0214] Example:

1250M−50M−(5/4*924.8M)=44M

[0215] where:

[0216] Fsig=50 MHz

[0217] FLO1=1250 MHz

[0218] FLO2=924.8 MHz

[0219] Fif=44 MHz

[0220]FIGS. 21 and 22 utilized this formula to derive the values enteredinto them to tune the exemplary cable TV signals “Frf”. For example thefirst column 2102 of the table lists the frequencies needed to tune asignal centered at 50 MHz (“Frf”) to a 44 MHz final IF (“IF-3”) . Totune a received channel centered at 50 MHz a first LO of 1,250 MHz(“LO-1”) is provided by a wide band, or coarse, PLL. This produces afirst IF of 1,200 MHz (“IF-1”) . Next utilizing 100 kHz tuning steps toadjust LO 2, it is set to 924.8 MHz (“LO-2”). Note this is not exactly925 MHz. Dividing the second LO by 4 in this instance yields 231.2 MHzfor a third LO (“LO-3”). When LO 3 is applied to the second IF of 275.2a third IF of 44 MHz (“IF-3”) is produced. This tuning arrangement isillustrated for received channels having a six MHz channel spacing ascan be seen from the line entitled “Frf”. In each case the coarse finetuning approach yields a third IF (“IF-3”) of 44 MHz.

[0221]FIG. 22 is an illustration of an alternative embodiment of thecoarse and fine PLL tuning method to produce an exemplary final IF of 36MHz. In this case as previously, a first IF (IF-1)is tuned to 1,200 MHzplus or minus 4 MHz. And second LO (LO-2) is close to 930 MHz, utilizinga small offset to yield a third IF of 36 MHz (IF-3). These predeterminedtuning frequencies are stored in a memory and applied when a command isgiven to tune a given channel. Alternatively an algorithm may besupplied to produce the tuning frequencies. It is understood that thesefrequencies are exemplary and other frequencies that utilize this methodare possible.

[0222] Thus, it can be seen that the interaction of course and fine PLLfrequencies are utilized to produce a third IF of 44 MHz. A second LO(LO-2) is maintained close to a frequency of 925 MHz to tune each of thechannels. However, it is slightly off by a very small tuning step of 100kHz. Note that the first IF (IF-1) is not always right at 1,200 MHz.Sometime it is off by as much as 4 MHz either above or below 1,200 MHz.This error will still result in signal transmission through a first IFfilter. The maximum error utilizing this scheme is plus or minus 4 MHz.

[0223] This method of PLL adjustment is described in more detail in U.S.patent application Ser. No. ______ filed ______, (B600:34015) entitled“System and Method for Coarse/Fine PLL Adjustments” by Pieter Vorenkamp,Klaas Bult and Frank Carr; based on U.S. Provisional Application No.60/108,459 filed Nov. 12, 1998 (B600:33586), the subject matter of whichis incorporated in its entirety by reference.

[0224] A coarse, and a fine PLL use a common reference frequencyoscillator. Local oscillator signals produced by the frequencysynthesizer's phase locked loops inject noise produced in the referencefrequency oscillator and the PLLs into a signal path through the PLLoutput. Noise injected can be characterized as either phase noise orjitter. Phase noise is the frequency domain representation of noisethat, in the time domain is characterized as jitter. Phase noise istypically specified as a power level below the carrier per Hertz at agiven frequency away from the carrier. Phase noise can be mathematicallytransformed to approximate a jitter at a given frequency for a timedomain signal. In a clock signal jitter refers to the uncertainty in thetime length between zero crossings of the clock signal. It is desirableto minimize the jitter produced in an oscillator circuit and transmittedthrough the signal chain into the signal path to prevent noisedegradation in the receiver path. Equivalently, any oscillator producinga stable output frequency will suffice to produce a reference frequencyfor the PLL circuitry.

[0225] Another obstacle to integrating an entire receiver on a singleCMOS chip has been the inability to fabricate a satisfactory filterstructure on the chip. As previously described, a multitude of unwantedfrequencies created through circuit non linearities are a major obstaclein achieving satisfactory receiver performance. Filtering is one methodof eliminating these unwanted spurious signals. An integrated filter'scenter frequency tends to drift, and needs calibration to maintainperformance. To successfully use filtering on chip, an auto calibrationloop is needed to center the filter response.

[0226]FIG. 23 is a block diagram of a dummy component used to model anoperative component on an integrated circuit chip.s According to oneaspect of the invention, a dummy circuit on an integrated circuit chipis used to model an operative circuit that lies in a main, e.g. RF,signal path on the chip. Adjustments are made to the dummy circuit in acontrol signal path outside the main signal path. Once the dummy circuithas been adjusted, its state is transferred to the operative circuit inthe main signal path. Specifically, as shown in FIG. 23, there is a mainsignal path 2201 and a control signal path 2202 on an integrated circuitchip. In main signal path 2201, a signal source 2203 is coupled by anoperative circuit 2204 to be adjusted to a load 2205. Main signal path2201 carries RF signals. Signal source 2203 generally represents theportion of the integrated circuit chip upstream of operative circuit2204 and load 2205 generally represents the portion of the integratedcircuit chip downstream of operative circuit 2204. In control signalpath 2202, a control circuit 2206 is connected to a dummy circuit 2207and to operative circuit 2204. Dummy circuit 2207 is connected tocontrol circuit 2206 to establish a feedback loop. Dummy circuit 2207replicates operative circuit 2204 in the main signal path in the sensethat, having been formed in the same integrated circuit process asoperative circuit 2204, its parameters, e.g., capacitance, inductance,resistance, are equal to or related to the parameters of operativecircuit 2204. To adjust operative circuit 2204, a signal is applied bycontrol circuit 2206 to dummy circuit 2207. The feedback loop formed bycontrol circuit 2206 and dummy circuit 2207 adjusts dummy circuit 2207until it meets a prescribed criterion. By means of the open loopconnection between control circuit 2206 and operative circuit 2204 thestate of dummy circuit 2207 is also transferred to operative circuit2204, either on a one-to-one or a scaled basis. Thus, operative circuit2204 is indirectly adjusted to satisfy the prescribed criterion, withouthaving to be switched out of the main signal path and without causingdisruptions or perturbations in the main signal path.

[0227] In one implementation of this dummy circuit technique describedbelow in connection with FIGS. 24a-c and FIGS. 25-27, operative circuit2204 to be adjusted is a bank of capacitors in one or more operativebandpass filters in an RF signal path, dummy circuit 2207 is a bank ofrelated capacitors in a dummy bandpass filter, and control circuit 2206is a phase detector and an on-chip local oscillator to which theoperative filter is to be tuned. The output of the local oscillator iscoupled to the dummy filter. The output of the dummy filter and theoutput of the local oscillator are coupled to the inputs of the phasedetector to sense the difference between the frequency of the localoscillator and the frequency to which the dummy filter is tuned. Theoutput of the phase detector is coupled to the dummy filter to adjustits bank of capacitors so as to tune the dummy filter to the localoscillator frequency. After the dummy filter is tuned, the state of itscapacitor bank is transferred, either on a one-to-one or scaled basis,to the operative filter. Since the capacitor bank in the dummy filterreplicates that of the operative filter, the frequency to which theoperative filter is tuned can be easily scaled to the frequency of thedummy filter.

[0228] In another implementation of the dummy circuit techniquedescribed below in connection with FIGS. 28 to 33, operative circuit2204 to be adjusted is a filter having a spiral inductor that has atemperature sensitive internal resistance. Dummy circuit 2207 has anidentical spiral inductor. Control circuit 2206 has a controllablevariable resistor in series with the inductor of dummy circuit 2207. Thecontrollable resistor is driven by a feedback loop to offset changes inthe internal resistance of the inductor of dummy circuit 2207. Operativecircuit 2204 has a similar controlled resistor in series with itsinductor to transfer the resistance value of the controllable resistorin control circuit 2206 to the resistor of the operative circuit 2204 inopen loop fashion.

Filter Tuning

[0229]FIG. 23a is a block diagram illustrating the use of a tuningcircuit outside of a signal path to tune bandpass filters present in areceiver. A tuning circuit 2302 utilizes a substitute or “dummy” filterstage 2310 to derive tuning parameters for a filter bank 2304 present ina signal path 2306. The tuning circuit utilizes a local oscillatorsignal 2308 available in the receiver to tune the dummy filter 2310 tothe center frequency of the local oscillator. Once tuned, the dummyfilters 2310 tuned component values that result in a tuned response atthe local oscillator frequency are scaled in frequency and applied tothe bandpass filter 2312. The filters are tuned at startup, and thetuning circuitry is turned off during normal operation. This preventsthe injection of additional noise into the signal path during operation.

[0230]FIG. 23b is a flow diagram of the tuning process in operationreceiver is initially powered up 2312 and local oscillator signalsgenerated by PLLs are centered at their design frequency 2314. Once thePLLs are locked their frequency is a known condition. Next substitutefilter tuning is initiated 2316 and performed. When finished a signal isreceived back from the filter tuning network indicating that it is ready2318. Information from the tuning network is copied to the receive pathfilter circuit 2320. Next the filter tuning circuit is turned off 2322disconnecting it from the filter circuit. In the embodiments of theinvention the narrow band PLL (2308, of FIG. 23a) is used as referencefrequency in the tuning circuit. However, it is understood that thistuning technique may be utilized with any readily available signal.

[0231] Returning to FIG. 23a, in an exemplary embodiment of theinvention a 925 MHz signal is directly available from the narrow bandPLL 2308. It is used to tune the dummy filter 2310 contained in thetuning circuit 2302 associated with the 1,200 MHz filter 2304. After thedummy filter is tuned to 925 MHz, frequency scaling is used to obtainthe proper component values for the 1,200 MHz filter response to becentered. The exemplary 925 MHz signal generated by the narrow band PLLis divided by 4 to yield a 231 MHz third LO signal utilized inadditional tuning circuitry.

[0232] Other divisions or multiplications may be equivalently used totune dummy filters. A second exemplary filter tuning circuit 2302 for a275 MHz filter contains a dummy filter 2310 that is tuned to a centerfrequency of 231 MHz. Once tuned, the component values used to centerthe 231 MHz dummy filter 2310 are scaled to yield a centered responsefor the 275 MHz filter 2304. At this point in time the tuning circuits2302 are switched off. It is especially important to turn off theexemplary tuning circuits on the 275 MHz filter since the 231 MHz signalused to tune its dummy filter falls in an exemplary 50-860 MHz band.

[0233] It is to be understood that any available frequency may be usedto tune a substitute filter so that another filter, that does not havean appropriate tuning signal present, may be tuned. This is done byscaling the component values of the tuned dummy filter to valuesappropriate for the filter not having the tuning frequency present.Tuning values obtained for a dummy filter may be applied to all filterspresent in a bank of filters having a common center frequency. Alsotuning values obtained for a dummy filter may be applied to multiplefilters present having differing center frequencies by applyingdiffering scaling factors. Finally multiple filters at differentlocations in a signal path that have common center frequencies may betuned by a common tuning circuit.

[0234] Capacitors disposed on an integrated circuit vary in capacitancevalue by as much as +/−20%. Thus, to provide a satisfactory receiverperformance a method of tuning integrated filters that removes thisvariation in capacitance is needed. In an LC filter circuit either aninductance or a capacitance can be tuned. However, inductors aredifficult to tune. Therefore, in the embodiments of the invention valuesof capacitance present in the filters are tuned. In tuning the exemplaryembodiments, one or more capacitors are switched in and out of an LCfilter circuit to tune it.

[0235] These capacitors are switched in and out of a filter circuitelectronically. Capacitors with the same dimensions are provided in abandpass filter and a dummy filter to provide satisfactory matchingbetween the devices. Switchable caps in the embodiments of the inventionare MOS caps that are all of the same value and from factor. However, itis to be recognized that other weighting of capacitor values could beprovided to achieve an equivalent function. For example, binary or 1/xweighted values of capacitors could be disposed in each filter toprovide tuning. In the embodiments of the invention a bank of fixedcapacitors and a bank of electronically tunable capacitors are provided.The adjustable capacitors in the exemplary embodiment represent 40% ofthe total capacitance provided. This is done to provide for the 120%variance in center frequency due to manufacturing variances. Toaccommodate other ranges of manufacturing variations or alternativetuning schemes any fraction or all of the capacitors may be switchable.It is also understood that any type of switchable capacitor, in additionto a MOS capacitor type may be utilized.

[0236]FIG. 24 is an exemplary illustration of a tuning process utilizingswitched capacitors. Filter responses shown at the bottom plot 2402illustrate a tuning of a dummy filter 2310 that is contained in a tuningcircuit 2302 of FIG. 23a. A frequency response being tuned in the uppergraph 2404 shows the tuning of the exemplary 1,200 MHz bandpass filter2304 of FIG. 23a. Initially none of the switched capacitors are appliedin a dummy filter circuit. This places the filter response initially2406 above the final desired tuned response frequency 2408. In thisexample capacitors are added until the filter response of the dummyfilter is centered about 925 MHz. However, the tuned response of the 925MHz dummy filter 2408 is not the desired center frequency of thebandpass filter in the signal path. The values used in to tune the dummyfilter would not tune the 1,200 MHz filter to the correct response.Frequency scaling is used to tune the desired response. This can beachieved because identical capacitors disposed on a chip are very wellmatched in value and parasitics. In particular capacitor matching iseasy to achieve by maintaining similar dimensions between groups ofcapacitors. In scaling a response to determine a capacitance to apply ina bandpass filter, identical inductance values have been maintained inthe dummy and bandpass circuits. Thus, only a scaling of the capacitorsis necessary. The frequency relation in the exemplary embodiment isgiven by the ratio: $\begin{matrix}{\frac{\quad_{1}}{\quad_{2}} \approx \sqrt{\frac{\left( L_{2} \right)\left( C_{2} \right.}{\left( L_{1} \right)\left( C_{1} \right.}}} & (5)\end{matrix}$

[0237] For this particular embodiment utilizing identical inductorvalues L₁=L₂. This reduces to: $\begin{matrix}{\frac{f_{1}}{f_{2}} \approx \sqrt{\frac{\left( C_{2} \right)}{\left( C_{1} \right)}}} & (6)\end{matrix}$

[0238] For the exemplary embodiment this is equal to 925/1200, or acapacitance ratio of 3:5. However, it is understood that other ratioswill allow tuning to be performed equivalently.

[0239] Returning to FIG. 23a various control signals applied to thetuning circuit are shown. In the event that the tuning is slightly offafter the tuning procedure, an offset control circuit is provided withinthe tuning circuit of FIG. 23 to move the tuning of the filters up ordown slightly by providing a manual means of adding or removing acapacitor. This control is shown by an “up/down” control line 2324 ofFIG. 23a. The exemplary tuning circuit of FIG. 23 is additionallyprovided with a “LO” 2308 tuning frequency to tune the dummy filter. The“10 MHz reference” signal 2326 is utilized as a clock in the tuningcircuit that controls the sequence of adding capacitors. The “reset”signal 2328 resets the tuning circuit for the next tuning cycle.

[0240]FIG. 25 is a block diagram of an exemplary tuning circuit. A resetsignal 2502 is utilized to eliminate all the capacitors from the circuitat power up by resetting a counter 2504 that controls the application ofthe switched capacitors. The reset signal may be initiated by acontroller or generated locally. This provides a known starting pointfor filter tuning. Next a filter figure of merit is examined todetermine iteratively when to stop tuning.

[0241]FIG. 26 illustrates the amplitude 2602 and phase 2604 relationshipin an LC filter tuned to its center frequency, fc. In tuning a filter toa center frequency two responses are available for examination.Amplitude and phase response are parameters that may be used to tune thefilter. For a wide band LC filter amplitude response 2602 is not theoptimal parameter to monitor. At the center frequency the top of theresponse curve is flat making it difficult to judge if the response isexactly centered. The phase response 2604 however, has a ratherpronounced slope at the center frequency. The steep slope of the phasesignal provides an easily discernable transition for determining whenthe center frequency has been reached.

[0242] Returning to FIG. 25, phase detection is used to detect when adummy filter 2506 has been tuned. An exemplary 925 MHz input from anarrow band PLL is input 2508 to a phase detector 2510. The phasedetector compares the phase of a signal input to a dummy filter 2508 toa phase of the output 2512 of that filter 2506. The phase detectorproduces a signal that is internally low pass filtered to produce a DCsignal 2514 proportional to the phase difference of the two inputsignals 2512, 2508. When tuned there is a 90 degree phase shift acrosscapacitors internal to the phase detector, that corresponds to 0 degreesof phase shift across the filter. Zero (0) degrees of phase shiftproduces a 0 volt output. Since it is known that with the capacitorsswitched out of the filter circuit 2506 that the center frequency of thefilter is high, the comparator 2516 following the low pass filter isdesigned to output 2518 a high signal that enables filter capacitors tobe switched in until the phase detector 2510 indicates no phasedifference is present across the filter 2506 at the tuned frequency.With a zero degree phase shift detected the comparator 2516 disables thecounter preventing any further capacitors from being switched into thefilter circuit.

[0243] The phase detector 2510 of the exemplary embodiment utilizes agilbert cell mixer 2512 and an integral low pan filter 2525 to detectphase. However, other phase detectors may be equivalently substitutedfor the mixer circuit. The 90° phase shift between an i port 2508 and aq port 2512 is being detected by the mixer. A 90° phase shift betweenthe i and the g signals in the mixer provides a 0 volt output indicatingthat those signals are in quadrature relation to each other. The signalsare shown as differential signals, however single ended signals mayequivalently be used.

[0244] The phase detector out 2514 is next fed into a comparator 2516that is set to trip on a zero crossing detected at its input. When azero crossing is encountered as the phase detector output approacheszero, the comparator latches and a counter 2504 is shut off and reset2518. The comparator function is equivalently provided by any standardcomparator circuit known by those skilled in the art.

[0245] The counter 2504 counts based on the 10 MHz reference clock 2524,although many periodic signals will suffice as a clock. As the counteradvances more filter capacitors are switched into the circuit. In theembodiments of the invention 15 control lines 2526 are used tosimultaneously switch the capacitors into the dummy filter and thebandpass filter bank. The control lines remain hard wired to bothfilters 2528, 2506, and are not switched off. However, once thecomparator 2516 shuts the counter 2504 off the tuning circuit 2530 isinactive and does not affect the band pass filter 2520 in the signalpath.

[0246]FIG. 27 is a schematic diagram showing the internal configurationof switchable capacitors in a differential signal transmissionembodiment of the dummy filter 2506 and the construction of the phasedetector 2510. A set of fifteen control lines 2526 are utilized toswitch fifteen pair of MOS capacitors 2702 on and off. The capacitorsare switched in and out by applying a given control signal to a virtualground point 2704 in this configuration. Thus, when the capacitors areconnected as shown the control signal is being applied at a virtualground. Thus, parasitic capacitances at this point will not affect thefilter 2506 performance. A gain producing LC stage 2706 of the dummyfilter is of a differential configuration and has its LC elements 2708connected in parallel with the MOS capacitors 2702.

[0247] Thus, with a capacitance ratio of 3:5 being utilized in theexemplary one line of embodiment a hard wired bus 2526 going to thedummy filter 2506 will switch in 5 unit capacitors, while the other endof the line that goes to the bandpass filter (2528 of FIG. 25) in thesignal path will switch in 3 unit capacitors.

[0248] In the mixer circuit that is used as a phase detector 2710 in theexemplary embodiment, differential image (“i”) signals I_(P) and I_(N)and differential quadrature (“q”) signals Q_(P) and Q_(N) are input tothe phase detector. A conventional Gilbert cell mixer configured as aphase detector 2710, as shown, has delay between the i port 2508 and qport 2512 to the output 2514. The i delay to the output tends to belonger due to the fact that it must travel through a greater number oftransistors than the q input to output path. Thus, even if i and q areexactly 90 degrees out of phase a DC offset tends to produced due to thepath length differences causing a phase error. To remedy this situationa second Gilbert cell mixer is duplicated 2710 and connected in parallelwith the first 2710. However, the i port and the q port connected to themixer 2712 are swapped to average out the delay thus tending to reducethe offset. This results in an almost 0° output phase error that isindependent of frequency. Other types of phase detectors and other meansof equalizing the delay, such as a delay line are understood by thoseskilled in the art to provide an equivalent function.

[0249] In the embodiment shown, the loss pass filter is implemented by asingle capacitor 2714 at each output. However, other equivalent methodsof achieving a low pass filter known to those skilled in the art areacceptable as well.

[0250] A method of filter tuning the advantageously uses the frequencysynthesizer output is fully described in U.S. patent application Ser.No. ______ filed ______ (B600:34013) entitled “System and Method forOn-Chip Filter Tuning” by Pieter Vorenkamp, Klaas Bult and Frank Carr;based on U.S. Provisional Application No. 60/108,459 filed Nov. 12, 1998(B600:33586), the subject matter of which is incorporated in itsentirety by reference.

[0251] Filters contain circuit elements whose values are frequency andtemperature dependent. The lower the frequency, the larger the size ofthe element required to realize a given value. These frequency dependentcircuit elements are capacitors and inductors. The fabrication ofcapacitors is not as problematic as the fabrication of inductors on anintegrated circuit. Inductors require relatively more space, and becauseof their size has a temperature dependent Q.

Compensation for Inductor Q Draft with Temperature

[0252]FIG. 28 is a plan view of a multi-track spiral inductor. Aninductor of this type is made from several long narrow strips of metalconnect in parallel and disposed upon an integrated circuit substrate. Amulti-track integrated spiral inductor tends to produce an inductancewith a high Q. High Q is desirable to achieve lower noise floors, lowerphase noise and when used in filters, a better selectivity. To reduceseries resistance and thus Q of a spiral inductor, wide track widths inthe spiral are used. However, when track width is increased beyond 10-15μm the skin affect causes the series resistance of a spiral inductor toincrease at high frequencies. Thus, Q is reduced even though a widetrack has been used. This trend tends to limit the maximum Q achievablein integrated spiral inductors.

[0253] An exemplary embodiment of the invention utilizes a spiralinductor that is wound with several narrow tracks disposed in parallelupon a substrate. By splitting an exemplary 30 μm wide track into two 15μm tracks disposed in parallel on the substrate, the inductor Q tends toincrease. Alternative embodiments of the invention by utilize singletrack spiral inductors or multiple track inductors containing one ormore tracks disposed in parallel upon a substrate. In the multiple trackinductors described, the tracks are joined together at the beginning ofa winding and again joined together at the end of the winding by aconductive material. An exemplary inductor suitable for integration isdescribed in more detail in U.S. patent application Ser. No. ______filed ______ (B600:______) entitled “Multi-Track Integrated SpiralInductor” by James Yung-Chieh Chang; based on U.S. ProvisionalApplication No. 60/117,609 filed Jan. 28, 1999 (B600:34072) and U.S.Provisional Application No. 60/136,654 filed May 27, 1999 (B600:34676).The disclosure thereof is incorporated herein in its entirety byreference thereto.

[0254] One or more spirals of metal have a series resistance associatedwith them. A spiral can be quite long, thus, the series resistance ofthe inductor is not negligible in the design of the circuit even with aparallel connection of tracks. As the temperature of the circuit rises,such as would occur after the initial power-up of an integrated circuit,the series resistance of the inductor increases, thus causing the Q todecrease. Circuitry is provided to continuously compensate for thisincreasing series resistance.

[0255] An inductor, or coil, has always been a fabrication problem inintegrated circuitry. Inductors are typically not used in integratedcircuits due to the difficulty of fabricating these devices and due tothe large amount of area required to fabricate them. A given inductancemay be realized by a single strip or metallic ribbon of a given widthand thickness suspended over a ground plane. A multiple track inductoralso requires more space than a simple track device.

[0256] It is a rule of thumb that the higher the frequency the smallerthe dimensions of the integrated circuit component required in a filterto achieve a given set of circuit values. A spiral inductor of the typedescribed in the embodiments of the invention allows an inductance to besatisfactorily fabricated on a CMOS substrate. Many alternativeembodiments of the spiral are known to those skilled in the art. Therealization of inductance required in any embodiment of the invention isnot limited to a particular type of integrated inductor.

[0257]FIG. 29 is an exemplary illustration of the possible effects ofinductor Q on filter selectivity in a parallel LC circuit, such as shownin 2706 of FIG. 27. The Q of a spiral inductor tends to be low. In orderto advantageously control the Q so that the maximum performance of anintegrated filter may be obtained, calibration of inductor Q is used.

[0258] The overall effect of this is that when a device with high seriesresistance and thus, low Q is used as a component in a filter that theoverall filter Q is low 2902. A high Q filter response is sharper 2984.The goal of a filter is to achieve frequency selectivity. The filterselectivity is the same electrical property as selectivity in the “frontend” of the receiver previously described. If the filter has a low Qfrequencies outside the pass band of the filter will not achieve asgreat of an attenuation as if the filter contained high Q components.The high degree of selectivity is required to reject the multitude ofundesirable distortion products present in a receiver that fall close tothe tuned signal. Satisfactory inductor dimensions and device Q havebeen obstacles in integrating filters on a CMOS substrate.

[0259] Prediction of the inductance yielded by the spiral is closelyapproximated by formula. However, prediction of the inductor's Q is moredifficult. Three mechanisms contribute to loss in a monolithicallyimplemented inductor. The mechanisms are metal wire resistance,capacitive coupling to the substrate, and magnetic coupling to thesubstrate. Magnetic coupling becomes more significant in CMOStechnologies with heavily doped substrates, because the effect ofsubstrate resistance appears in parallel with the inductor. The firstfour or five turns at the center of the spiral inductor contributelittle inductance and their removal helps to increase the Q. In spite ofextensive research inductors implemented in CMOS possess Qs afterlimited to less than five.

[0260]FIG. 30 is an illustration of a typical filter bank 3002 utilizedin embodiments of the invention for filtering I and Q IF signals 3208.Band pass filters utilized in the embodiments of the invention have acenter frequency f_(c) and are designed to provide a given selectivityoutside of the pass bond. The exemplary filters 3002 also incorporategain. Gain and selectivity are provided by a transconductance stage withan LC load resulting in an active filter configuration that gives thefilter response shown. Over temperature the filter response degrades asindicated. This degradation is attributed to inductors. With the spiralinductors utilized in the embodiments of the invention the gain of thisfilter stage is substantially determined by the Q or quality factor ofthe inductor. The Q is in turn substantially determined by the seriesresistance of the metal in the spiral of the inductor. The Q decreasesas temperature increases causes an increase in inductor seriesresistance. The decrease in Q with increasing temperature adverselyaffects the filter characteristics. As can be seen in 306 at FIG. 30 asthe temperature increases from 50° C. 3004 to 100° C. 3006 overall gaindecreases, and selectivity is degraded.

[0261]FIG. 31 is a diagram of an exemplary differential transconductancestage 3102 with an LC load 3104. This figure comprises elements of oneof the filter gain stages that are a part of one of the filters thatcomprise the filter bank 3002 of FIG. 30. Two forms of the LC load'sequivalent circuit are shown in the FIG. 3106, 3108. Resistor R(T) hasbeen added 3106 to account for the series resistance of inductor L thattends to increase in direct proportion to the temperature of theinductor. The circuit may in turn be represented in parallel form 3108to yield an equivalent response using the elements L′ and R′(T). Amethod of compensating for the parallel R′(T) is desirable. It is doneby increasing the Q of the filters with Q enhancement, and bystabilizing the enhanced value of Q obtained over the range oftemperatures encountered in circuit operation. First the implementationof Q enhanced filters is explained.

[0262]FIG. 32 shows a transconductance stage 3102 with an LC load 3104that is provided with Q enhancement 3202 and Q compensation overtemperature 3206. Q enhancement 3202 tends to increase the circuit Qthus, increasing the frequency selectivity of the circuit. A Qenhancement is provided by the transconductance element's G_(m), 3202connected as shown. Addition of this transconductance element isequivalent to adding a negative resistance 3024 that is temperaturedependent in parallel with R′(T). This negative resistance tends tocause cancellation of the parasitic resistance thus, tending to increasethe circuit Q. The details of Q enhanced filters are disclosed in moredetail in U.S. patent application Ser. No. ______ filed ______(B600:______) entitled, “New CMOS Differential Pair LinearizationTechnique” by Haideh Khorramabadi; based on U.S. Provisional ApplicationNo. 60/136,115 filed May 26, 1999 (B600:34678), the subject matter ofwhich is incorporated in this application in its entirety by reference.Once an improved Q is achieved it is desirable to maintain it over therange of temperatures encountered in circuit operation with temperaturecompensation circuitry 3206.

[0263] Due to a large positive temperature coefficient inductor qualityfactor (Q) is proportional to temperature. As temperature increases theresistance in the spiral increases, degrading the Q. The addition oftransconductance from the G_(m) stage 3102 tends to increase the Q ofthe filter. However, the effects of temperature on quality factor tendsto cause wide gain variation tending to need further improvement. In anembodiment of the invention for a temperature range from 0 to 100° C., Qand gain vary +/−15% in an unenhanced filter. In an embodiment with a Qenhanced filter, the Q and gain variation is doubled. In multiple stagesof filtering used in the embodiments, over 20 db of gain variation isthus encountered over temperature with the Q enhanced filters. Thisresults in an unacceptable change in the conversion gain of thereceiver. A further means of reducing the variation in Q (and thus gain)over temperature is desirable 3206.

[0264]FIG. 33 shows a method of stabilizing inductor Q over temperature3206. This method advantageously uses a DC calibration loop 3202 and adummy inductor 3304 to control the value of inductor series resistanceR(T) and a resistive element R(1/T) 3314 to produce a net constantresistance. Thus, Q induced variation in filter response due totemperature are controlled. This method advantageously does not requirethe use of any high frequency signals in the tuning process. An inductor3306 as utilized in the filters of FIG. 30's filter bank 3002 with itsassociated series resistance R(T) is shown as an element in atemperature compensation circuit 3208. An electronic device thatsupplies a variable resistance 3310 of an amount inversely proportionalto temperature is added into the circuit 3314. The decreasing resistanceof the additional resistance 3314 with increasing temperaturecounteracts the increasing resistance of the inductor's seriesresistance R(T). In the circuit diagram this decreasing resistance isshown schematically as R(1/T) This resistance is provided by the activeresistance of a PMOS transistor biased accordingly 3314. However anydevice capable of producing the desired resistance characteristicdescribed above is an acceptable substitute.

[0265] A PMOS resistor is used in two places 3312,3314 to place thecontrol element 3314 in the circuit and remove the control circuit 3208from a main circuit 3308. In the embodiment shown, the PMOS transistor'sgate to source connection is placed in series with the spiral inductor3306 of the LC circuit 3308 making up an active filter stage. The activefilter stage is controlled from a remotely located control circuit 3208that contains a duplicate PMOS resistor 3312 and inductor 3304. Inductor3304 is advantageously fabricated with the same mask pattern as used forinductor 3306. The control circuitry 3208 is not a part of the filtercircuitry 3308 in order to prevent undesirable interactions with theradio frequency signals present in the filter. In the control circuitshown, the active resistor 3312 in series with the spiral inductor 3304is duplicated remotely from the filter circuit 3308. To communicate thecontrol signal 3316 the gate of the PMOS resistor 3312 is coupled to thegate of the PMOS resistor in the filter 3314.

[0266] The control circuit provides a conventional constant current anda conventional constant voltage source function to maintain a constantcurrent through and voltage across the dummy spiral inductor 3304duplicated in the control circuit. An exemplary constant current andconstant voltage source is shown 3302 incorporating a dummy inductor3304. However, any circuit that maintains a constant voltage across, andcurrent through the inductor 3304 in the control circuit 3208 issufficient for the design.

[0267] As gate voltage 3316 changes to maintain the constant current andvoltage across the inductor in the control circuit 3304, the gatecontrol signal 3316 is simultaneously fed to the LC filter stage 3308PMOS transmitter 3314 to control the resistance, and thus the Q, of theinductor in the filter circuit 3308.

[0268] An exemplary constant current and voltage source is illustrated3302 comprising dummy inductor 3304. A temperature independent voltagereference V_(ref) is established by resistor R and conventional currentsources I. Amplifier A's negative input is connected to the voltagereference, and its positive input is connected to a symmetrical pointbetween an identical current source and the dummy inductor. The outputof amplifier A is fed into the gate of the transistor functioning as avariable resistor 3312. The constant voltage drop over temperature atthe node V_(ref) is compared to the voltage at the positive amplifierterminal. The amplifier controls the resistance of the PMOS transistorso that a constant current and constant voltage are maintained acrossthe dummy inductor.

[0269] The calibration of inductor Q is described in more detail in U.S.patent application Ser. No. ______ filed ______ (B600:34014) entitled“Temperature Compensation for Internal Inductor Resistance” by PieterVorenkamp, Klaas Bult and Frank Carr; based on U.S. ProvisionalApplication No. 60/108,459 filed Nov. 12, 1998 (B600:33586), the subjectmatter of which is incorporated in its entirety by reference.

Communications Receiver

[0270]FIG. 34 is a block diagram of a communications network utilizing areceiver 3402 according to an exemplary embodiment of the invention. Acommunications network, such as a cable TV network 3404, capable ofgenerating signals provides radio frequency signals 3406 over the airwaves, through a cable or other transmission. A receiver front end 3408next converts the RF single ended signal to a differential signal. Areceiver front end, or a Balun may be used to convert a single endedsignal 3406 to a differential signal or vise versa 3410. The receiverblock which contains an exemplary embodiment of the invention nextconverts the differential radio frequency signal 3410 to a differentialintermediate frequency (IF) 3412. The IF signal 3412 is next converteddown to PC and demodulated into a base band signal 3414 by a demodulator3416. At this point the base band signal 3414 is suitable forpresentation to the video input of a television receiver, the audioinputs to a stereo, a set top box, or other such circuitry that convertsthe base band signal into the intended information output.

[0271] The communication system described is contemplated to provide thefunction described above in one or more circuit assemblies, integratedcircuits or a mixture of these implementations. In particular, the RFfront end 3408 may be integrated in a single chip with receiver 3402.Alternatively, the front end and receiver may be implemented asindividual integrated circuits, on any suitable material such as CMOS.

[0272] In addition, the receiving system described utilizes additionalexemplary embodiments that incorporate one or more transmitters and oneor more receivers to form a “transceiver” or “multiband transceiver.”The transceiver contemplated may transmit and receive on differingfrequencies or the same frequency with appropriate diplexer, transmitreceive switching or functionally equivalent circuitry.

[0273] The frequency bands and modulation described in the specificationare exemplary with the inventions not being limited in scope to anyparticular frequency band or modulation type.

Receiver Front End-programable Attenuator and LNA

[0274] To achieve a low noise figure what is left out of the circuit isoften as important as what is included in it to achieve a low noisefigure. A circuit containing few components in desirable since eachcomponent in a circuit adds to noise generated in the circuit. Switchesare often included early in a signal path to switch in attenuatorsections, reducing the level of a signal present. The reduction insignal level is necessary to prevent a following receiver circuit frombeing over driven into distortion.

[0275] Additionally, the circuit described as a front end circuit mayalso be employed as an automatic gain control (“AGC”) amplifier. The AGCamplifier may advantageously be used at any point in the signalprocessing chain where an adjustable gain and adjustable attenuationaccording to an external control signal is desired. In one specificembodiment, a control signal 4302 representative of the signal level ofbase band signal 3414 (FIG. 34) is fedback from block 3416 to RF frontend 3408. By way of example, control signal 4302 could be formed bysampling the sync pulses of the base band television signal andaveraging the amplitude of the sync pulses over a period of time.

[0276] Advantageously, the present invention has eliminated the need forswitches, reducing a major contributor to increased noise figure. In anintegrated switchless programmable attenuator and low noise amplifieronly two elements are present in the signal path to contribute to thenoise figure. First an attenuator is present in the circuit path. Thenext element in series with the attenuator in the signal path is adifferential pair low noise (LNA) amplifier. In the differential pairnoise figure is lowered by introducing a sufficient bias current toincrease a transconductance g_(m) associated with the amplifier. Theincreased g_(m) decreases the noise contribution of the differentialpair.

[0277] By eliminating the need for switches it is possible to integratethe programmable attenuator and LNA onto a single CMOS integratedcircuit. An additional advantage can be realized in using an integratedprogrammable attenuator and LNA as a “front end” of an integratedreceiver. A single integrated circuit can be economically fabricated onCMOS that contains an entire tuner circuit including the front end andthe tuner. Alternatively, the front end and tuner circuits may be onseparate interconnected substrates.

[0278]FIG. 35 is an illustration of the input and output characteristicsof an integrated switchless programmable attenuator and low noiseamplifier 3502. Attenuator/amplifier 3502 simulates a continuouslyvariable potentiometer that feed a linear amplifier. As thepotentiometer setting changes the signal level at the input to theamplifier changes, and the output of the amplifier changes accordingly.The exemplary embodiment is a two radio frequency (RF) port device—theinput port 3504 is configured to receive a single ended input signalfrom a source 3508 and the output port 3506 is configured to present adifferential signal. In the single ended input configuration oneterminal upon which a signal is carried is above ground reference 3510.In the differential output configuration the signal is divided andcarried on two terminals above ground reference 3510.

[0279] In the exemplary embodiment multiple control signals 3512 areapplied to the integrated switchless attenuator and LNA 3502. Forexample these signals are used to program the attenuator to variouslevels of attenuation, and for an output smoothness control.

[0280] In the exemplary embodiment the differential output 3506advantageously tends to provide noise rejection. In a differentialoutput configuration, the signal at one terminal is 180° out of phasefrom the signal at the other terminal and both signals are ofsubstantially equal amplitude. Differential signals have the advantagethat noise that is injected on either terminal tends to be canceled whenthe signal is converted back to a single ended signal. Such common modenoise is typically of equal amplitude on each pin and is typicallycaused by radiation into the circuit from external sources, or it isoften generated in the circuit substrate itself. Advantageously, thepresent invention uses differential signal transmission at its output.It should be noted that in alternate embodiments of the invention, thata signal ended output can be produced from the differential signal byvarious techniques known in the art. Also, equivalently a differentialinput may be substituted for the single ended input shown.

[0281]FIG. 36 is a functional block diagram of the integrated switchlessprogrammable attenuator and low noise amplifier circuit. This embodimentillustrates how it is possible to eliminate switches that would berequired in a conventional attenuator and LNA.

[0282] A resistive attenuator 3601 is configured as a ladder circuitmade up of resistors configured as multiple pi sections 3602. A methodof selecting resistor values such that a constant impedance is presentedto the signal source is accomplished as is conventionally known in theart. An exemplary embodiment utilizes an R/2R configuration. Each pisection 3602 of the attenuator 3601 is connected to one input to adifferential pair amplifier 3603. The other input to amplifier 3603 isgrounded. The resulting attenuation produced at the output 3604 iscontrolled by the number of differential amplifier stages that areturned on and the degree to which they are turned on.

[0283] Individual amplifiers 3603 are turned on or off by tail-currentgenerators 3605 associated with each stage 3603, respectively.Generation of the tail currents is discussed in more detail below inconnection with FIGS. 44a and 44 b. In FIG. 36 a zero or one is used toindicate if the corresponding tail-current generator 3605 is turned onor off, that is whether or not a tail-current is present. For example, azero is used to show that no tail-current is present and thecorresponding generator 3605 is turned off. A one represents atail-current generator 3605 that is turned on rendering thecorresponding amplifier 3603 functional. The zeroes or ones are providedby the control lines 3512 of FIG. 35 in a manner described in moredetail in FIG. 43. All of the individual amplifier outputs 3506 aredifferential. Differential outputs 3506 are tied in parallel with eachother. The resulting output 3604 is the parallel combination of the oneor more amplifiers 3608,3610,3612 that are turned on. In an exemplaryembodiment of the circuit 55 amplifiers have been implemented, withvarious combinations turned on successively. By using tail currents toselectively turn amplifiers 3603 on and off, the use of switches isavoided.

[0284] In this configuration any combination of amplifiers 3603 could beturned on or off to achieve a given attenuation before amplification ofthe signal. However, in a exemplary embodiment of the circuit, adjacentpairs of amplifiers are turned on and off. Groupings of amplifiers inthe on state can be of any number. In an embodiment ten contiguousamplifiers are turned on. The attenuation is adjusted up or down byturning an amplifier tail current off at one end of a chain ofamplifiers, and on at the other to move the attenuation in the desireddirection. The exemplary circuit is controlled such that a group ofamplifiers that are turned on slides up and down the chain according tothe control signals 3512 of FIG. 35.

[0285] Any number of amplifiers 3603 can be grouped together to achievethe desired resolution in attenuation. By using the slidingconfiguration, input signals 3614 that are presented to attenuator pisections 3602 whose amplifiers are not turned on do not contribute tothe output signal 3604. It can be seen from FIG. 36 that the signalstrength of the output is dependent upon where the grouping ofgenerators 3605 are turned on.

[0286]FIG. 37 is a simplified diagram showing the connection 3702 ofmultiple attenuator sections 3602 to the output 3604. An attenuator 3601is made up of multiple pi sections 3602 cascaded together. Each pisection consists of two resistances of 2R shunted to ground, with aresistor of value R connected between the non grounded nodes. Tap points3702 are available at the nodes of the resistor R. In FIG. 37 the firstset of nodes available for tap points in the first pi section would benodes 3706 and 3708. After cascading all of the pi sections to form aladder network, a variety of tap points are available, these are notedas node numbers 3706-37150 in FIG. 37. A path from the input 3614 to anyof the tap points, or nodes on the ladder network yields a known valueof attenuation at the output 3604. If multiple tap points aresimultaneously connected to the attenuator, the resulting attenuation isthe parallel combination of each connection. The combined or averageattenuation at the output terminal can be calculated mathematically or,it can be determined using circuit simulation techniques available incomputer analysis programs.

[0287] In addition it can be seen from FIG. 37 that by providingmultiple tap points on a ladder network that in effect a slidingmultiple contact action can be implemented contacting a fixed number ofcontacts, for any given position of the simulated slide 3716. The slide3716 is implemented electronically in the embodiments of the inventionThe average attenuation by contacting a fixed number of these tap points3706-3715 will increase as the slide or switch is moved from the left tothe right on the ladder network. For example, minimum attenuation willbe present when the slider 3716 contacts the force tap points3706,3707,3708,3709 at the far left of the ladder network 3601. Themaximum attenuation will be achieved when the slider 3716 is positionedto contact tap points 3712,3713,3714,3715 at the far right of thenetwork. In the exemplary embodiment 4, contacts are shown, however, inpractice any number of contacts may be utilized.

[0288] Mechanical switches are noisy. Mechanical switches are alsounreliable and difficult to integrate on a semiconductor device.Returning to FIG. 36, in order to be able to integrate a switchingfunction, and to eliminate mechanical parts, a predetermined number ofattenuator taps are switched to the output by using tail currentswitching of differential amplifiers 3603,3605. The differentialamplifiers have the advantage of being able to be switchedelectronically with low noise and reliability. The differentialamplifiers also provide the opportunity to introduce a gain into thecircuit thereby increasing the signal strength available at the outputto produce a low noise amplification. The gain achieved depends upon thenumber of amplifiers switched in. By changing the values of resistancein the ladder network and also by increasing or decreasing the number ofamplifier stages that are turned on, the resolution of the attenuatorcan be varied to suit the needs of the system that an integratedswitchless programmable gain attenuator and LNA is used in.

[0289]FIG. 38 is an illustration of an exemplary embodiment showing howthe attenuator 3601 can be removed from the circuit, so that only theLNAs or differential stages 3605 are connected. Reference numerals 3801to 3816 each represent a differential amplifier 3603 and a generator3605 in FIG. 36. In the 0 dB attenuation case shown the signal strengthof the output would be equal to the gain of the parallel combination ofthe four amplifiers that are turned on 3801,3802,3803,3804. The fouractivated amplifiers are indicated by a “1” placed on the circuitdiagram. In an exemplary embodiment in which the sliding tap arrangementis used such that a given number of amplifiers are always turned on theconfiguration of FIG. 38 is necessary such that zero decibels ofattenuation can be achieved when the required number of amplifiers arealways turned on.

[0290] In an exemplary embodiment according to FIG. 38, a full 14 dBgain from a combination of ten amplifiers is seen when a ten tapconfiguration is used with the top set to the 0 dB attenuation position.As the attenuation is “clicked” so that one amplifier at a time isswitched, a 1 dB per pi section attenuator is placed in series with anamplifier, a full 1 dB of attenuation is not seen/click. In a graph ofthe control voltage versus attenuation curve this would be seen as achange in slope after the tenth amplifier is switched in. After the 10thamplifier is switched in the curve will show a 1 dB/adjustment step.

[0291]FIG. 39 shows an exemplary attenuator circuit used to achieve 1dB/step attenuation. Each resistive pi section 3602 makes up one step.The characteristic impedance of the embodiment shown is 130 ohms. Usingcalculation methods well known in the art of attenuator design a pi padhaving a characteristic impedance of 130 ohms may be realized utilizingseries resistors R_(s) of 14 ohms or parallel or shunt resistors of1,300 ohms R_(p).

[0292]FIG. 40 illustrates an exemplary embodiment of an attenuator forachieving a finer resolution in attenuation. In this embodiment aresolution of 0.04 dB/tap is achieved. In the embodiment shown eachseries resistor R_(s), connected between the shunt resistors in theladder network has a string of series resistors connected in parallelwith it. Each interconnection point between the added resistors 3402provides a tap point that provides a finer adjustment in attenuationvalues.

[0293] In implementing an integrated, switchless, programmableattenuator and low noise amplifier, calculating the overall gain of aparallel combination of amplified and attenuated signals is analyticallycomplex to calculate. For example, consider an embodiment utilizing 10differential pair amplifiers in the output, connected to 10 differenttap points. Ten signals receiving varying attenuations are fed intoindividual differential pair amplifiers. Gain of the amplifiers variesaccording to an adjustment for monotonicity. The amplified signals arethen combined in parallel to yield the output signal.

[0294] Tail currents in the differential output amplifiers are not allequal. The tail currents determine the gain of a differential pair, andare adjusted to provide a specific degree of monotonicity. Thus, thegain of each of the differential pair amplifiers varies across the 10interconnected amplifier. The attenuation varies since each tap is takenat a different point to be fed into each of the differential amplifiers.In such an arrangement it would be expected that the middle signal linewould represent the average, yielding an approximate figure for theattenuation and gain of the combination of 10 signal lines. However,this is not the result. Through the use of computer simulation thebehavior of this network has been simulated. In simulating behavior ofthis network it is found that the first tap predominates in defining aresponse from the sum of the 10 taps. The first tap has the leastattenuation and this yields the predominant signal characteristics.

[0295] In an embodiment utilizing 10 sliding taps the amplifier gain isa constant 14 dB. The attenuator range is from 0-25 dB in 1 dB steps.This yields an overall range of −11 dB to +14 dB for the combination ofattenuator and amplifiers.

[0296]FIG. 41 illustrates the construction of the series and parallelresistors used an integrated attenuator. In this embodiment all of theresistors used are 130 ohms. This is done to control the repeatabilityof the resistor values during fabrication. Ten of these resistors areconnected in parallel to yield the 13 ohm resistor used as the seriesattenuator element R_(s) of FIG. 39. Ten of these 130 ohm resistors areconnected in series to yield 1,300 ohms to realize the parallelresistance legs R_(p) of FIG. 39 of the attenuator. Building theattenuator from unit resistors of 130 ohms also, provides improvedmatching. By matching resistor values in this method amiability isminimized to that of the interconnections between the resistors. Thisallows the ratio between series and parallel resistances to remainconstant from pi section to pi section 3602 in the ladder network thatmakes up the attenuator 3601 of FIG. 36.

[0297]FIG. 42 is an illustration of an exemplary embodiment utilized toturn on each of the differential amplifiers. This arrangement produces amonotonically increasing output verses control voltage 4202. In thisillustration, five amplifiers 4204-4208 grouped together make up theelectronically sliding tap arrangement. Numbers on the illustrationindicate the fractions of tail-currents relative to the full value usedto turn on each amplifier. Amplifiers are partially turned on at theends of the group. Gradual turn on of the amplifiers at the ends of thegroup is done to control overshoots and undershoots in the amplifiergain. These over shoots and under shoots are seen upon the applicationof a control voltage applied.

[0298] Varying a smoothness control provided in a programmableattentuator and LNA to one extreme yields good linearity in thefrequency response but overshoots in gain with increases in controlvoltage. Varying the smoothness control to the other extreme yields avery smooth gain verses control voltage curve with more nonlinearity.The optimum value for the smoothness control yields a value ofmonotonicity that is the maximum that the system can tolerate in theform of data loss throughout the circuit.

[0299] If all five amplifiers of FIG. 42 were turned on with the fullvalue of tail-currents, the gain versus control voltage curve would beas shown in the solid line 4210. By not fully turning on some of thedifferential pair amplifiers the overshoot and undershoot in the gainversus control voltage curve may be minimized. With the tail-currentsconfigured on the sliding tap as shown in FIG. 42, the gain versuscontrol voltage curve will appear as shown by the dotted line 4202. Inthis configuration, the middle three amplifiers have their tail-currentsfully turned on with the remaining two amplifiers at the beginning andend of the chain only having their tail-currents half turned on.Equivalently, other weighing of total currents may be used to achievesubstantially the same effect.

[0300] A plot of gain versus control voltage for the entire integratedswitchless programmable attenuator and low noise amplifier wouldpreferably appear as a staircase over the entire control voltage range.By controlling the turn on of the tail-current, the non-monotonicity ofthe gain versus the control voltage curve is reduced so that the gainmonotonically increases with the application of an increasing controlvoltage to yield the desired stair step shape response, where FIG. 42illustrates one “step” 4202 in the response. Non-monotonicity in gainversus control voltage is not a time dependent phenomenon. The shape ofthe curve tends to depends on the physical implementation of a circuitand a switching arrangement for turning tail-currents on and off.

[0301] Non-monotonicity is an undesirable characteristic tends todegrade overall systems performance. In receiving QAM data thedegradation is seen as a loss in received data. By improving themonotonicity characteristic of an amplifier linearity of the amplifieris degraded. Gradual switching of the tail-currents causes somedifferential pairs to only partially turn on. Differential pairs thatare partially turned on introduce more nonlinearities into the circuitoutput than a fully turned on differential pair.

[0302] A transistor that is only partially turned on is only capable ofhandling a smaller signal than one that is more fully turned on. Atransistor that is only partially turned on receiving a large inputsignal over drives the transistor producing a distorted output. Thus, bygradually turning on the tail-currents in some of the differential pairamplifiers, the linearity tends to be degraded, however, thisdegradation in linearity allows a monotonically increasing gain versuscontrol voltage curve to be achieved.

[0303] Monotonic increase of gain versus control voltage tends toimprove system performance. In the case of the QAM television signalbeing transmitted through the amplifier a view of a QAM constellationwould actually be seen to wiggle with tail-currents of all differentialpair amplifiers simultaneously and fully turned on. With gradualtail-current switching, the constellation is not seen to wiggle, anddata is not lost. The problem with the non-monotonicity causing theconstellation to wiggle is that each time an attenuator value isswitched into the circuit QAM data tends to be lost, thus degradingoverall system performance of the signal transmitted through thecircuit.

[0304] As part of an exemplary embodiment's operation, an automatic gaincontrol (AGC) 3512 of FIG. 35 would be generated as one of the controlsignals by external receiver circuitry to adjust the input signal levelpresented to the receiver. This AGC control voltage would be fed into acontrol voltage input 3512 to select a value of attenuation through thecircuit assembly. It is desirable to switch the attenuator such thatwhen the attenuation is adjusted, the data is not lost due to theswitching period. In an exemplary embodiment of the present invention itis necessary to switch a maximum of 0.04 dB per step in attenuationvalue.

[0305]FIG. 43 is an illustration of an embodiment showing how individualcontrol signals 4301 used to turn on individual differential pairamplifiers are generated from a single control signal 4302. There aremany ways to generate control signals to turn on the differential pairamplifiers, individual control lines may be utilized, or a digital toanalog converter may be used to transform a digital address to an analogcontrol voltage.

[0306] In the embodiment of FIG. 44 to generate the control signalsresistors 4304 are connected in series between a power supply voltageand ground to create a series of reference voltages at eachinterconnecting node. The voltages at each node between the resistors isthe reference input for one of a series of comparators 4306. Thereference input of the comparator connects to a node providing thereference voltage setting. The other input of the comparator isconnected to the control voltage 4302. When the value of the controlvoltage exceeds that of the reference voltage for a given comparator thecomparator goes from a zero state to a one state at its output. The zerostate is typically zero volts and the one state is typically somevoltage above zero. The voltage generated to produce the logic one stateis such that when applied to a gate of a transistor making up thecurrent tail 4308 it is sufficient to turn on the differential pair ofamplifiers that constitute the low noise amplifier (LNA) controlled bythat current tail.

[0307] As can be seen from FIG. 43, all the LNA amplifiers set to beactivated with a control voltage of the current setting will be turnedon. In this arrangement simply increasing the control voltage simplyturns on more LNA amplifier stages. Additional circuitry is required todeactivate previously activated amplifiers such that only a fixed numberof amplifiers remain turned on as the control voltage increases. This isdone so that the sliding potentiometer function can be implemented withthis circuit.

[0308]FIG. 44 is an illustration of an embodiment of one of theindividual comparator stages 4308 of FIG. 43 used to turn on or offindividual LNA amplifier stages. In the integrated switchlessprogrammable attenuator and low noise amplifier the circuitry used toactivate individual cells is duplicated at each attenuator's tap pointand interconnected so that a sliding tap can be simulated using a singlecontrol voltage, V_(ctr) 4302. In describing a cell's operation it isconvenient to start with the control voltage 4302 that is being appliedto achieve a given attenuation value.

[0309] To illustrate the comparators operation, a control voltage isapplied to each of a series of comparators, as is shown in FIG. 43. Thecircuit of FIG. 44 makes up one of these comparators. FIG. 44 shows thecontrol voltage as V_(ctr), and the reference voltage as V_(ref). Thesevoltages are applied to the gates of a differential pair of transistors(Q1 Q2) . The circuit in FIG. 44 surrounding Q1 and Q2, functions as acomparator with low gain. The gain of the comparator is kept low tocontrol the switching on and off the tail-currents of the low noiseamplifiers.

[0310] In FIG. 44 when the control voltage input V_(ctr) passes thereference level set at V_(ref) the amplifier with its reference setclosest to, but less than VCtr remains deactivated. (The n+1 amplifierswhere V_(ctr) has not exceeded V_(ref) remain turned off, untilactivated by V_(ctr).) First the comparator output “current (cell n)”goes high. When “current (cell n)”, which is connected to the gate ofQ15, goes high it switches the transistor on. Transistors Q16 and Q17are used to deactivate the adjoining current mirror circuit. Amplifier,Amp_(n) is turned off by shunting current away from the current mirror4402, shutting off the tail current Q15. Thus, the current amplifiercell with a comparator that has just been tripped remains turned off.

[0311] Comparator output signal “next (cell n+10)” is the opposite stateof “Current (cell n)”. A cell 10 cells away is turned off by the controlsignal “next (cell n+10)”. These cells have not yet had theircomparators tripped by the control voltage present on their inputs. Thusthe bottom of the sliding tap is pushed up and down by the controlvoltage, V_(ctr). In this state transistors Q16 and Q17 in the next 10cells are not conducting current away from the current mirror. Thisallows the current tails of each amplifier, Q15 to conduct causingamplifier Amp_(n) to be turned on in each of the 10 cells.

[0312] Note that as a larger number of cells are grouped together, forsimultaneous turn on, a larger number of differential amplifier cells inthe integrated switchless programmable attenuator and low noiseamplifier are required to achieve the same attenuation range.

[0313] Once the control voltage has been exceeded for a given cell, thedefault state for all the previous amplifiers Amp_(n) is to be turnedon, unless the cell is deactivated by either Q1 or Q2 being activated.

[0314] The input signal “previous (from cell n−10)” deactivatesamplifier cells when it is in the high state. This signal is suppliedfrom the tenth previous identical comparator.

[0315] In FIG. 44 a provision for adjusting the abruptness of amplifiergain is provided. Transistors Q3 and Q10 are being used as variableresistors. These variable resistors are used to change the gain of thecomparator. Varying the gain of the comparator allows the abruptness inthe overall amplifier gain to be controlled. Putting a high voltage on“smoothness control” causes the drain of Q5 and Q6 to be shortedtogether. The gain is reduced and a very gradual transition betweenstates is provided by doing this.

Receiver Frequency Plan and Frequency Conversion

[0316] Returning to FIG. 19 a block diagram illustrating the exemplaryfrequency conversions utilized in the embodiments of the invention. AnRF signal 1906 from 50 MHz to 860 MHz that is made up of a plurality ofCATV channels is mixed 1916 down by a first LO (LO₁) 1912 that rangesfrom 1250 MHz to 2060 MHz, depending upon the channel tuned, to a firstIF signal 1918 that is centered at 1,200 MHz. This 1,200 MHz first IFsignal is passed through a first filter bank 1912 of cascaded band passfilters to remove undesired spurious signals. The first frequencyconversion in the receiver is an up conversion to a first intermediatefrequency 1918 higher than the received RF frequency 1906. The firstintermediate frequency is next mixed 1932 down to a second IF 1922.

[0317] A second local oscillator signal at 925 MHz (LO₂) 1904, is usedto mix 1932 the first IF 1918 down to a second IF 1922 signal centeredat 275 MHz. A second bank of band pass filters 1934 removes spuriousoutputs from this second IF signal 1922, that have been generated in thefirst two frequency conversions.

[0318] A third frequency conversion 1924, or the second down conversionto the third IF 1926 is accomplished with a third LO (LO₃) 1930 of 231MHz. A third filter 1936 removes any spurious responses created by thethird frequency conversion and any remaining spurious responses thathave escaped rejection through the previous two filter banks. This thirdband pass filter 1936 may have its response centered at 36 or 44 MHz. A44 MHz IF produced by the 231 MHz LO is used in the United States whilea 36 MHz IF is used in Europe. The LO₃ is adjusted accordingly toproduce the 36 MHz IF. The local oscillator's signals are advantageouslygenerated on chip in the described embodiments. However, the receiverimplementation need not necessarily be limited to on chip frequencygeneration.

Local Oscillator Generation

[0319]FIG. 45 is a block diagram illustrating the exemplary generationof local oscillator signals utilized in the embodiments of theinvention. The frequency plan utilized in the embodiments utilizes apure third local oscillator signal (LO₃) 1930, created by directsynthesis 4502 that falls within the band of received signals. The firsttwo local oscillator LO₁ 1902, LO₂ 1904 signals are generated usingindirect synthesis achieved by a phase locked loops 4504,4506. The thirdlocal oscillator signal (LO₃) uses direct synthesis, to divide thesecond local oscillator down to create the third local oscillator (LO₃).The indirect synthesis of the first and second LOs utilizes a frequencyreference 4508 provided by a 10 MHz crystal oscillator. The 10 MHzcrystal oscillator utilizes the previously disclosed differential signaltransmission and a unique design that advantageously tends to provide anextremely low phase noise reference signal. The first local oscillator(LO₁) 1902 is produced by wide band tuning. The second local oscillator(LO₂) 1904 is produced by narrow band tuning. The exemplary embodimentsadvantageously utilize a narrow band tuning circuit and method toachieve frequency lock in an exemplary narrow band PLL.

Narrow Band VCO Tuning

[0320]FIG. 46 is a schematic of a PLL having its VCO controlled by anembodiment of the VCO tuning control circuit. A VCO tuning controlcircuit is provided to tune a VCO that is contained in an exemplarynarrow band PLL that generates a 925 MHz local oscillator signal. Thisdevice makes use of a temperature and process dependent window ofvoltage ranges to optimally choose a range of valid control voltages forthe PLL. The control circuit uses a window to center a varactor diode'stuning range by adding or removing capacitance, thus tending to avoidgross varactor non-linearities. The circuit tends to mitigate dead bandconditions and tends to improve loop stability over process andtemperature variations.

[0321] A VCO integrated on a chip can be up to ±20% off in its frequencyrange. Immediate calibration at power up is done to center the varactordiodes that provide a variable tuning capacitance to the middle of thevaractor diode's tuning range. This is done by switching in capacitorsand monitoring loop voltage. To center the VCO's tuning capacitancerange of the varactors, the embodiments of the invention immediatelycalibrate the VCO by adding or removing capacitance. Switchingcapacitors in or out of the circuit centers the varactor's capacitanceinto the middle of the VCO's tuning range. To monitor centering of thevaractors a window comparator is used to look at the state of a controlvoltage that is used to tune the VCO. The window comparator determineswhen the control voltage is within its desired range.

[0322]FIG. 46 illustrates the VCO tuning control circuitry 4604 appliedto a conventional PLL 4602. PLL 4602 comprises a crystal oscillator 4606that inputs a stable frequency to a programmable 4608 reference divider4610 that outputs a frequency 4612 based upon the reference frequency tothe input of a phase detector 4614, a second input 4616 to the phasedetector is the current output of a VCO 4618. The phases of the twoinputs 4612,4616 are compared and a DC value representing the phasedifference is output 4620 to the input of a charge pump 4622. The outputof the charge pump is fed into a low pass filter 4624. The output of lowpass filter 4624 is fed into the control voltage input of the VCO 4618.The VCO outputs an image and quadrature signal 4626 at a frequency asset by the frequency select line 4608.

[0323] The voltage controlled oscillator 4618 is conventionallyconstructed, and comprises a variable capacitance used to tune theoutput frequency. VCO 4618 additionally comprises a series of switchablecapacitors utilized to center the tuning range of the variablecapacitance elements comprising the VCO. The switchable capacitors arecontrolled by signals emanating from the VCO tuning control circuitry4604. The control signals 4628 are routed from tuning register 4630 toVCO 4618.

[0324] The VCO tuning control circuitry utilizes a control signal 4632taken from low pass filter 4624. Control voltage 4632 is input to thepositive inputs of a first comparator 4634 and the positive input of asecond comparator 4636. The negative inputs of comparators 4634 and 4636are coupled to DC reference voltages V1 and V2. Comparator 4634 outputssignal lsb and comparator 4636 output signal msb. Voltages V₁ and V₂ setthresholds to form a sliding window which monitors the state of theclosed PLL by monitoring voltage at low pass filter 4624. Controlvoltage 4632 is taken as the voltage across a capacitor in the low passfilter that induces a zero in the loop filter 4624. Thus, the controlvoltage is a filtered version of the control voltage of the PLL loop,and thus tends to have eliminated spurious components present on the VCOcontrol line.

[0325] Signals msb and lsb are fed in parallel to an AND gate 4640 andan exclusive NOR gate 4642. The output of exclusive NOR gate 4642 is fedinto the D input of a DQ flip-flop 4644. The Q output of the flip-flopis fed into an AND gate 4646, whose output is in turn fed into the clockinput of a 6-bit bi-directional tuning register 4630.

[0326] Returning to AND gate 1940 its output is fed into the shift leftor right input port of the 6-bit bi-directional tuning register 4630.Additionally, DQ flip-flop 4644 receives a reset signal based on theoutput of low pass filter 4624. flip-flop 4644 is also clocked by asignal based on the divided reference oscillator signal 4612.

[0327]FIG. 47 is a process flow diagram illustrating the process oftuning the VCO with an embodiment of a VCO control circuit. Initiallythe control voltage (4632 of FIG. 46) is evaluated to see if it fallswithin a predetermined window 4702. If the voltage is within the desiredrange, the time it has remained so is determined 4704. The PLL tends tobe in a state of lock when the control voltage applied to the VCO hasremained unchanged for a predetermined period of time. If the voltagedoes not remain in range for the predetermined time, the process isreinitiated by looping back to the beginning. If the control voltageremains in the range for the predetermined time, the loop is deemed inlock, and the process is ended 4712.

[0328] Returning to block 4702, if the control voltage is out of range adecision is made 4706 based on, wether the control voltage is above orbelow the desired range. If the control voltage is greater than thecontrol voltage range, a capacitance is removed from the VCO circuit4708. The process flow is routed to the beginning of the process, wherethe control voltage is again reevaluated 4702.

[0329] Returning to block 4706, if the control voltage is below thedesired range a capacitor is added 4710. Next, the process routes theflow back to the beginning of the process where the control voltage isreevaluated 4702.

[0330] The VCO tuning control circuitry 4604 of FIG. 46 functions tocarry out the process of FIG. 47. If the voltage of the loop liesoutside the window defined by the threshold voltages V_(1 and V) ₂. Theclock input to the 6-bit bi-directional tuning register 4630 is enabled.This register function may be provided by a conventional circuitry knownin the art to provide this function and is not limited to the circuitrydepicted. A “lock time out” circuit 4648 of FIG. 46 is reset on therising edge of the clock signal to the 6-bit bi-directional tuningregister 4630 of FIG. 46. The “lock time out” circuit is conventionallyconstructed and is not limited to the components depicted in FIG. 46.

[0331] If control voltage 4632 exceeds the upper threshold set by thecomparators, zeros are shifted through the register 4630. A zero voltagedecreases the capacitance in the VCO tuning circuitry by switching out acapacitance controlled by one of the 6 control lines 4628.Alternatively, any suitable number of control lines may be used otherthen the exemplary six. This shifting of values in a register allows oneof six exemplary capacitor switch control lines to be activated ordeactivated, an evaluation made and another line activated ordeactivated so that the previous tuning setting is not lost. Thisfunction may be implemented by passing a value (on or off) down a lineof capacitors by shifting or by activating a capacitor associated with agiven line and then a next capacitor without shifting the capacitancecontrol signal.

[0332] If the control voltage 4632 is less than the lower thresholdvoltage of the comparator 4634 1 s are shifted through the 6-bitbi-directional tuning register. The 1 s increase the capacitance appliedin the VCO tuning circuit by switching in a capacitance controlled byone of the 6 control lines 4628.

[0333] Once control voltage 4632 enters the predetermined valid range ofoperation as set by voltages V₁ and V₂ the shift register 4630 isdisabled. At this time the locked time out circuit 4648 is enabled. Ifthe lock time out circuit remains enabled for the predetermined timeperiod, that satisfies the in lock condition for the PLL, the clock tothe DQ flip-flop 4644 is disabled, thus disengaging the control circuit.The functions described in this paragraph are constructed from standardlogic components known to those skilled in the art, and are not limitedto those components depicted in FIG. 46.

[0334] A more detailed description of the VCO tuning scheme is providedin U.S. patent application Ser. No. ______ filed ______ (B600:36226)entitled “System and Method for Narrow Band PLL Tuning” by Ralph Duncanand Tom W. Kwan; based on U.S. Provisional Application No. 60/136,116filed May 26, 1999 (B600:34677), the subject matter which isincorporated in its entirety by reference. Once the fine, or narrow bandPLL has been tuned such that is has been locked its frequency may beused in conjunction with the frequency generated by the coarse PLL toprovide channel tuning as previously described for the coarse/fine PLLtuning of FIGS. 21 and 22.

Receiver

[0335]FIG. 48 is a block diagram of a first exemplary embodiment of areceiver. FIGS. 48, 51, 52, 53 and 54 are embodiments of receivers thatutilize band pass filters and image reject mixers to achieve imagerejection that tend to reduce the distortion previously described. Theembodiments advantageously convert an input signal (1906 of FIGS. 19,48, 51, 52, 53 and 54) to a final IF frequency (1914 of FIGS. 19 48, 51,52, 53 and 54) by processing the input signal substantially as shown inFIG. 19. Image rejection is measured relative to the signal strength ofthe desired signal. The strength of the unwanted image frequency ismeasured in units of decibels below the desired carrier (dB_(c)). In theexemplary embodiments of the invention an image frequency rejection of60 to 65 dB_(c) is required. In the embodiments of the invention thisrequirement has been split more or less equally among a series ofcascaded filter banks and mixers following the filters. The filter banks1912,1934 provide 30 to 35 dB_(c) image rejection and complex mixers4802,4806 used provide an additional 30 to 35 dB_(c) of image rejectionyielding an overall image rejection of 60 to 70 dB_(c) for thecombination. The use of complex mixing, advantageously allows therejection requirements on the filters to be relaxed. First, a channel ofan input spectrum is centered about a first IF frequency.

[0336]FIG. 49 is an exemplary illustration of the frequency planningutilized in the embodiments of the invention for the reception of CATVsignals. The frequency spectrum at the top of the FIG. 4902 illustratesexemplary received RF signals ranging from 50 to 860 MHz 4904. Thereceived RF signals are applied to a band pass filter 4921 to eliminateout of band distortion products Image1 4906. The frequency planadvantageously utilizes a trade off between image rejection achievableby filters and mixers at different frequencies. The processing of thefirst IF and the second IF have many features in common and will bediscussed together in the following paragraphs.

[0337] For example, the second mixer 4802 and second bank of IF filters4834 of FIG. 48 achieve 35 dB and 35 dB of image rejection,respectively. The third mixer 4806 and the third IF filter bank 1936 ofFIG. 48 achieve 25 dB and 40 dB of image rejection respectively. Thelast distribution reflects the fact that at the lower third IF frequencythe Q of the filters tend to be lower, and the image rejection of themixers tend to be improved at lower frequencies.

[0338] For example, returning to FIG. 48, a signal 1906 in the 50 to 860MHz range is up converted by mixer 1916 and LO2 1908 to 1,200 MHz IF-11918. The presence of LO-2 1904 at 925 MHz that is required to mix thesignal IF-1 1918 down to the 275 MHz IF-2 1922 has an image frequencyImage2 (4908 as shown in FIG. 49) at 650 MHz. The filter Q of the 1,200MHz center frequency LC filter 1912 causes Image2 to undergo 35 dB ofrejection thus, attenuating it. To achieve 70 dB of image rejectionanother 35 dB of rejection must be provided by the second mixer (4702 ofFIG. 48) that converts the signal from 1,200 MHz to 275 MHz.

[0339] Continuing with FIG. 48, the same structure as described in thepreceding paragraph is again encountered, but at a lower frequency forthe second IF 4914. Image rejection of the 275 MHz filter (1934 of FIG.48) is less due to its lower Q and the fact that the image frequencyImage3 4912 is spaced only 88 MHz 4910 from the signal IF-2 4914. In theprevious first IF stage the image frequency Image2 4908 was spaced 550MHz 4918 from the signal IF-1 4916, providing better image attenuationby filter stop bands. In this situation 25 dB of selectivity can beachieved in the filter, requiring 40 dB of rejection in the mixer toachieve at least 65 dB of attenuation of Image3.

[0340] Phase matching at lower frequencies is more accurate allowingbetter image rejection to be obtained from the third mixer. The methodof trading off filter selectivity against mixer image rejection atdifferent frequencies advantageously allows a receiver to successfulintegrate the filters on chip with the desired image frequencyrejection. This process is described in detail in the followingparagraphs.

[0341] Returning to FIG. 48, it is desired to up convert a channelreceived in this band of signals 1906 to a channel centered at anintermediate frequency of 1,200 MHz 1918. A local oscillator 1908produces frequencies from 1,250 MHz to 2060 MHz. For example, a channelcentered at 50 MHz is mixed with the local oscillator set at 1,250 MHzto produce first IF frequency components 1918 at 1,200 MHz and 1,300MHz. Only one of the two frequency components containing identicalinformation produced by the mixing process is needed; the low side 1,200MHz component is kept. Filtering 1912 tends to remove the unneeded highside component and other desired signals.

[0342] Choosing the first IF 1918 to be centered at 1,200 MHz makes thefirst IF susceptible to interference from a range of first imagefrequencies from 2,450 MHz to 3,260 MHz (4906 as shown in FIG. 49),depending upon the channel tuned. The lower image frequency of 2,450 MHzresults from the first IF of 1,200 MHz being added to the lowest firstLO present at 1,250 MHz to yield 2,450 MHz. The highest image frequencyresults from the first IF of 1,200 MHz being added to the highest firstLO of 2,060 MHz to yield 3,260 MHz as the highest first image. Choosingthe first IF 1918 at 1,200 MHz yields image frequencies (4906 of FIG.49) that are well out of the band of the receiver. The result tends toplace undesired frequencies far down on the filter skirts of filterspresent in the receiver, attenuating them.

[0343] After a channel is up conversion to a first IF 1918 of 1,200 MHz,it is next filtered by a bank of 3 LC band pass filters 1912 each havingits response centered at 1,200 MHz in the embodiment. These filters inconjunction with the second mixer 4802 provide 70 dB of image frequencyrejection (4908 of FIG. 49). Filters are advantageously integrated ontothe CMOS substrate. An LC filter comprises inductors (or coils) andcapacitors. An inductor implemented on a CMOS substrate tends to have alow Q. The low Q has the effect of reducing the selectivity and thus theattenuation of signals out of band.

[0344] The attenuation of signals out of band can be increased bycascading one or more filters. Cascading filters with identical responsecurves has the effect of increasing the selectivity, or furtherattenuating out of band signals. The embodiments of the inventionadvantageously incorporate active g_(m) stage filters 1912,1934 toincrease selectivity and provide circuit gain to boost in band signalstrength. Three cascaded active LC filters implemented on a CMOSsubstrate yield a satisfactory in band gain, and provide approximately35 dB of out of band image signal rejection in the embodiment described.However, the filters need not be limited to active LC filters, othercharacteristics and passive filters are contemplate equivalents.

[0345] The remaining 35 dB of image frequency rejection needed must beachieved in the other circuitry. Hence, differential I/Q mixers4802,4806 are advantageously used to achieve this approximate 35 dB ofadditional image rejection required in the first IF.

[0346]FIG. 50 is a block diagram illustrating how image frequencycancellation is achieved in an I/Q mixer. An I/Q mixer is a devicepreviously developed to achieve single side band signal transmission. Itis one of three known methods for eliminating one of two side bands.This type of mixer is able to transmit one signal while eliminating orcanceling another signal. An I/Q mixer advantageously possesses theproperties of image frequency cancellation in addition to frequencyconversion. For example, returning to FIG. 48, a second LO 1904 of 925MHz is used to create the down conversion to a second IF 1922 of 275MHz, while rejecting image frequencies from the previous frequencyconversion by LO1 1908.

[0347] The I/Q mixers are implemented in several ways in the invention.However the overall function is maintained. An interconnection ofcomponents that achieves I/Q mixing is illustrated in the exemplary I/Qmixer 4802 shown in FIG. 48.

[0348] First an input signal 1918 is input to a mixer assemblycomprising two conventional mixers 4828, 4830 of either a differential(as shown) or single ended construction.

[0349] Local oscillator signals 1904, that need not necessarily bebuffered to achieve I/Q mixing, are applied to each mixer. The localoscillator signals applied to each mixer are of the same frequency, but90 degrees out of phase with each other. Thus, one signal is a sinefunction, and the other is a cosine at the local oscillator frequency.The 90 degree phase shift can be generated in the I/Q mixer orexternally. In the circuit of FIG. 48 a conventional poly phase circuit4832 provides the phase shift and splitting of a local oscillator signalgenerated by PLL2 4806.

[0350] Two IF signals, an I IF signal and a Q IF signal, are output fromthe mixers and fed into another conventional poly phase circuit 4834.Thepoly phase circuit outputs a single differential output IF signal.

[0351] Returning to FIG. 50, the I/Q mixer uses two multipliers5002,5004 and two phase shift networks 5006,5008 to implement atrigonometric identity that results in passing one signal and cancelingthe other. The trigonometric identity utilized is:

cos(2πf _(RF) t)cos(2πf _(LO1) t)±sin(2πf _(RF) t)sin(2πf _(LO1)t)=cos[2π(f _(RF) −f _(LO1))t]  (7)

[0352] where

[0353] f_(RF) is an input signal 5010

[0354] f_(LO1) is the first LO 5012

[0355] The signals produced and blocks showing operations to createsignal transformation of these signals to yield the desired final resultis shown in FIG. 50. The process makes use of a hardware implementationof the trigonometric identities:

sin(u)sin(v)=½[cos(u−v)−cos(u+v)]  (8)

[0356] and

cos(u)cos(v)=½[cos(u−v)+cos(u+v)]  (9)

[0357] By applying these trigonometric identities to the signals createdby the two mixers, the product of the sine waves 5014 is:

½[cos(2πf _(LO1) t−2πf _(RF) t)−cos(2πf _(LO1) t+2πf _(RF) t)]  (10)

[0358] and the product of the cosines 5016 is:

½[cos(2πf _(LO1) t−2πf _(RF) t)+cos(2πf _(LO1) t+2πf _(RF) t)]  (11)

[0359] Thus, two frequencies are created by each multiplication. Two ofthe frequencies have the same sign and frequency, so that when they areadded together 5018 the resultant signal is a positive sum 5020. Theother frequency created cancels itself out 5022. The sum frequencycomponent created by the product of the sines is a negative quantity.The same sum frequency component created by the multiplication of thecosines is positive and of equal magnitude. Thus, when these signals areadded together one frequency component, the difference, that is presentin each signal has twice the amplitude of the individual signals and thesecond, sum frequency created is of opposite polarity of the othersignal created and cancels out when the signals are added together.Thus, the difference frequency is passed to the output while the sumfrequency component is canceled.

[0360] The implementation of this trigonometric identity by a circuit isvery useful for canceling image frequencies. As shown in FIG. 4 signal,S and image signal I are equally spaced by the IF frequency from thelocal oscillator frequency. The signal frequency would be represented bythe term (2πf_(LO1)t−2πf_(RF)t) and the image frequency would berepresented by (2πf_(LO1)t+2πf_(RF)t). In the embodiments of theinvention, the phase shifting and summing functions are performedutilizing standard polyphase or other circuits known in the art.

[0361] Mathematically exact cancellation can be achieved. However, realcircuit components are not able to achieve exact cancellation of theimage frequency. Errors in phase occur in the circuitry. A phase errorof 3° can yield an image frequency suppression of 31.4 dB_(c) and aphase error of 4° can yield an image frequency suppression of 28.9dB_(c). These phase errors tend to be achievable in an integratedcircuit on CMOS. To attempt to achieve the entire 70 dB_(c) of imagerejection tends to be undesirable, thus necessitating the filters. Forexample, to achieve 59 dB_(c) of image frequency rejection a phase errortending to be of no more than 0.125° in the mixer would be allowable.

[0362] By combining image frequency rejection achievable by an LC filterimplemented in CMOS with an I/Q mixer's image rejection properties,properties that tend to be achievable in a CMOS integrated circuit, arequired image frequency rejection is obtained. Additionally, thefrequency of a first up conversion has been advantageously selected toplace an image frequency of a first LO well down the filter skirts of a1,200 MHz LC filter bank, thus achieving the desired image frequencyrejection.

[0363] Returning to FIG. 48, buffer amplifiers 4810 are used torecondition the amplitudes of LO signals 1908,1904,1930 that drive theI/Q ports of mixers 4802,4806. A distance of several millimeters acrossa chip from where LOs are generated 4504,4506,4508,4502 to where it isapplied at the mixers 1916,4802,4806 tends to require reconditioning ofthe slopes of the local oscillator signals. Buffering also tends toprevent loading of the PLLs 4504,4806.

[0364] Eliminating any preselection filtering requiring tunable bandpass filters is desirable. To do this image frequency response and localoscillator (LO) signals are set to fall outside of a received signalsbandwidth. The first signal conversion tends to eliminate anyrequirements for channel selectivity filtering in the receiver frontend. Because of the integrated circuit approach to this design it isdesirable to locate an LO outside of the signal bandwidth to reducedistortion created by the interaction of the received signals and thefirst local oscillator signals.

[0365] An approximately 35 dB of out-of-band channel rejection in thefirst IF stage's filter 1912 is insufficient. The additional 35 dB ofselectivity provided by a mixer 4802 increases selectivity. However, itis desirable to mix down a received signal as quickly as possible. Thisis desirable because at lower frequencies filters tend to have betterselectivity than at the higher IF frequencies. By converting a receivedsignal to as low a frequency as possible as quickly as possible betterfiltering tends to be obtained. Two frequency down conversions are nextperformed.

[0366] Filters are available that will achieve a better rejection thanan LC filter at a given frequency, for example a SAW filter. Whilebetter filtering of the intermediate frequencies could be obtained witha filter such as a SAW filter at a higher frequency, a fully integratedreceiver would not be achievable. A SAW filter is a piezoelectric devicethat converts an electrical signal to a mechanical vibration signal andthen back to an electrical signal. Filtering is achieved through theinteraction of signal transducers in the conversion process. A filter ofthis type is typically constructed on a zinc oxide (ZnO₂), a materialthat is incompatible with integration on a CMOS circuit utilizing asilicon (Si) substrate. However in alternative embodiments of theinvention, SAW or other filter types known in the art including externalLC filters are contemplate embodiments. In particular, a hybridconstruction utilizing receiver integrated circuit bonded to a hybridsubstrate and filters disposed on the substrate is contemplated.

[0367] Returning to the frequency plan of FIG. 49, there is an imageresponse (Image2) 4908 associated with the second local oscillatorsignal (LO₂) 4920. Returning to the embodiment of FIG. 48, this Image2signal occurs at f_(LO2)−f_(IF2)−925 MHz−275 MHz, which is 650 MHz. Ifthere is a signal of 650 MHz at the receiver's input 4808 it is possiblethat a 650 MHz signal will be mixed down to the second IF frequency(IF₂) (1922 of FIG. 48) causing interference with the desired receivedsignal which is now located at the second IF frequency. To reduceinterference from this signal the receiver has been designed to producegreater than 65 dB of rejection of Image2 by the mechanism previouslydescribed for the 1,200 MHz LC filter bank 1912 of FIG. 48.

[0368] Returning to FIG. 48, the third IF is next generated. The thirdLO 1930 is created by direct synthesis. The divide by 4 block 4802creates a 231 MHz third LO (LO₃) consisting of I and Q signals requiredto mix the 275 MHz second IF 1922 down to the third and final IFfrequency of 44 MHz 1926. A second down conversion to the 275 MHz thirdIF is used in the design. If a 1,200 MHz first IF signal were downconverted directly to 44 MHz a local oscillator signal of 1156 MHz(1,200 MHz−44 MHz) would be required. A resulting image frequency forthis local oscillator would be at 1,112 MHz (1,200 MHz−88 MHz). A 1,112MHz image would fall within the band of the 1,200 MHz LC filter. Thus,there would be no rejection of this image frequency from the first IF'sfilter since it falls in the pass hand. Therefore, the intermediatefrequency conversion to a second IF of 275 MHz is used to reduce theeffects of the problem.

[0369] The 231 MHz third LO 1936 falls close to the center of thereceived signal band width 1906. With the three frequency conversions ofthe design the third LO necessarily falls within the received signalband. This is undesirable from a design standpoint. This is because anyspurious responses created by a third local oscillator signal fallwithin the received signal bandwidth. The present embodiment of thisinvention advantageously minimizes these undesirable effects.

[0370] In generating the third LO signal of 231 MHz, typically a phaselock loop containing a voltage controlled oscillator would be used.However, these frequency components tend to be primary generators ofspurious products that tend to be problematic. The present embodimentsof the invention advantageously avoids the use of a PLL and theattendant VCO in producing the third LO signal 1930 at 231 MHz. A divideby 4 circuit 4802 utilizes two flip-flops that create the I and Q thirdLO signals 1930 from the 925 MHz second LO 1904. This simple directsynthesis of the third LO tends to produce a clean signal. The reducedgeneration of distortion within the signal band tends to be important inan integrated circuit design where all components are in close physicalproximity. If a PLL were used to generate the 231 MHz signal an externalloop filter for the PLL would be utilized, providing another possiblepath for noise injection. By elegantly generating this third LO, thatnecessarily falls within the received signal bandwidth, noise andinterference injection through the substrate into the received signalpath tends to be minimized.

[0371] LC filter tuning 4812,4814,4816 in the embodiment isadvantageously performed at startup of the chip. A “1,200 MHz filtertuning” circuit 4812 tunes the 1,200 MHz low pass filters 1912; a “275MHz filter tuning” circuit 4814 tunes the 275 MHz low pass filter 1934;and a “44/36 MHz filter tuning” circuit 4816 alternatively tunes a finalLC filter 1936 to one of two possible third IF frequencies (44 MHz or 36MHz) depending upon the application. Alternatively, in this embodiment,the filtering of the third IF frequencies is done by an external filter4818. This external filter may have a saw device or other type of filterthat provides satisfactory filtering of the third IF frequency.

[0372] As previously described, the filter tuning circuits4812,4814,4816 utilize tuning signals based on the PLL2 signal 4806,with the “44/36 MHz filter tuning” circuit utilizing the PLL2 frequencydivided by four 4802. However, the tuning signals selected may vary. Anyor all of the PLLs 4804,4806,4802 or reference oscillator 4808 may beused to generate a filter tuning signal. Also a single frequency can beused to tune all filters with the appropriate frequency scaling applied.In tuning the LC filters, first the chip is turned on and PLL2 4806 mustlock. PLL2 must first lock at 925 MHz as previously described. A VCO inthe PLL 4806 is centered by adjusting its resonant circuit with tunablecapacitors as previously described.

[0373] Once the PLL2 is adjusted to 925 MHz a write signal is sent outto indicate that a stable reference for filter tuning is available. Oncea stable 925 MHz reference for tuning is available the 1,200 MHz filter,the 275 MHz filter tuning previously described takes place. Once thefilter tuning is finished the filter tuning circuitry sends out a signalover an internal control bus structure, linking the receiver to acontroller indicating that the tuning has finished. The receiver is nowready to select and tune a channel.

[0374] Frequency tuning of received channels is accomplished in theembodiment with a coarse and fine PLL adjustment as previouslydescribed. The tuning is performed in such a way that there is always athird IF present at the output during the tuning process. PLL1 4804 isthe coarse tuning PLL that tunes in 10 MHz steps. PLL2 4806 is the finetuning PLL that tunes in 100 KHz steps. Exemplary tuning steps can bemade as small as 25 KHz. A 100 kHz step is used for QAM modulation, anda 25 KHz step is used for NTSC modulation.

[0375] At the input of the tuner each exemplary channel is separated by6 MHz. PLL1 jumps in tuning steps of 10 MHz. Therefore, + or −4 MHz isthe maximum tuning error. If the filters used had a narrow band passcharacteristic this tuning approach tends to become less desirable. Forexample, if the filter bandwidth was one channel, 6 MHz, wide and thefirst IF could be 1204 MHz or 1196 MHz. Thus, the selected channel wouldnot be tuned. The bandwidth of the cascaded filters in the first IFstrip is approximately 260 MHz. The bandwidth of the filters centered at275 MHz in the second IF strip is approximately 50 MHz. The bandwidthsare set to be several channels wide, a characteristic thatadvantageously takes advantage of the low Q in the LC filters built onthe chip. The two PLLs guarantee that a third IF output is alwaysobtained. The first PLL that tunes coarsely must tune from 1,250 to2,060 MHz, a wide bandwidth. PLL2, the fine tuning PLL, must tune from +to −4 MHz, which tends to be easier to implement.

[0376]FIG. 51 shows a second exemplary embodiment of the invention. Thisembodiment is similar to the embodiment of FIG. 48, however iteliminates the first IR reject mixer (4802 of FIG. 48). Theapproximately 35 dB of image rejection that has been eliminated due tothe removal of the IR reject mixer is made up by increased filterrejection provided by a 1,200 MHz LC filter bank 5101. The IR rejectmixer is replaced with a conventional differential mixer 5104. The IOrequired is a single differential LO signal 5106 rather than thedifferential I and Q signals previously described. Better filters areused or alternatively an additional series of three 1,200 MHz LC filters1912 for a total of six cascaded filters 5101 to provide sufficientimage rejection are provided. This design provides the advantage ofbeing simpler to implement on an integrated circuit.

[0377] If a higher Q or better filter selectivity is realized on theintegrated circuit 65 dB of image frequency rejection at 650 MHz isrequired. In an alternate embodiment of the invention the third downconversion can be accomplished in a similar manner by eliminating thethird I/Q mixer 4806 and increasing the selectivity of the 275 MHzfilter bank 5102. The mixer 4806 is replaced with a conventional mixerrequiring only a single differential third LO.

[0378]FIG. 52 shows a third alternate embodiment of the invention thattends to provide continuous tuning of the filter over temperature, andtends to more accurately keeps the response curve of the filter centeredon the desired frequency. This embodiment of the invention preserves theseparation of I 5202 and Q 5204 signals through the second IF stage5206. In the third frequency conversion stage 5208 the I and Q signalsare transformed into I′, {overscore (I)}, Q, and {overscore (Q)}signals. This alternate embodiment of the invention relies on a“three-stage poly phase” 5210 to provide image cancellation. Theadvantage of using a gyrator in place of dual LC filter bank 5212 isthat a close relationship between I and Q tends to be maintainedthroughout the circuit. The phase relationship at the output of thegyrator filter tends to be very close to 90°. If an LC filter isutilized there is no cross-coupling to maintain the phase relationshipas in the gyrator. In the LC filter configuration complete reliance uponphase and amplitude matching is relied upon to maintain the I and Qsignal integrity. The gyrator circuit has the additional advantage oftending to improve the phase relationship of signals initially presentedto it that are not exactly in quadrature phase. For example, an I signalthat is initially presented to the gyrator that is 80° out of phase withits Q component has the phase relation continuously improved throughoutthe gyrator such that when the signals exit the gyrator quadrature phaseof 90° tends to be established between the I and Q signals, such as in apolyphase circuit element. This present embodiment of the inventionprovides the additional benefit of being easily integrated onto a CMOSsubstrate since the gyrator eliminates the inductors that an LC filterwould require. Filter timing and frequency generation utilize themethods previously described.

[0379]FIG. 53 is a block diagram of an exemplary CATV tuner thatincorporates an embodiment of the present invention. The exemplaryembodiments of the receiver are for terrestrial and cable televisionreception of signals from 50 to 860 MHz. Television signals in thisexemplary band are frequency QAM or NTSC modulated signals. A receiveras described performs equally well in receiving digital or analogsignals. However, it is to be understood that the receiver architecturedisclosed will function equally well regardless of the frequencies used,the type of transmission, or the type of signal being transmitted. Withregard to signal levels input to the receiver, the dynamic range of thedevices used in the receiver may be adjusted accordingly. Thus, in awide-band receiver distortion products are particularly problematic. Thereceiver disclosed in the exemplary embodiments of the present inventiontends to advantageously reduces interference problems created by thistype of distortion.

[0380] In the exemplary embodiments of the invention signals input tothe receiver may range from +10 to +15 dB_(m). Where, zero dB_(m)=10log(1 mV/1 mV) . It should be noted that in the case of a cabletransmitting the RF signals, that an attenuation envelope impressed onthe signals will have a downward or negative slope. This downward ornegative slope is a result of a low pass filter characteristic of thecoaxial cable. This effect may be compensated for by introducing a gainelement in the signal chain that has positive slope, to compensate forthe negative slope resulting from cable transmission.

[0381] In a wide band receiver designed to process signals received overmultiple octaves of band width, this transmission characteristic canpresent a problem. For example, in the cable television band going from50 to 860 MHz it is possible for distortion products created by thelower frequency signals in this band width to fall upon one of thehigher tuned frequencies, for example 860 MHz. In a multi octaveband-width receiver harmonic signals are problematic since they alsofall within the receiver band-width, and cannot be low pass filteredout. If a channel at one of the higher frequencies is the desired signalthat the receiver is tuned to, the low pass filter characteristic of thecable, or transmission medium, reduces the strength of this desiredtuned signal relative to the lower frequency untuned signals. Because ofthe relatively greater strength of the lower frequency signal, thestrength of the distortion products generated by them, are comparable instrength to the desired tuned signal. Thus, these distortion productscan cause a great deal of interference with the desired received signalwhen one of their harmonics coincidentally occurs at the same frequencyas the tuned signal.

[0382] The frequency plan of this tuner allows it to be implemented in asingle CMOS integrated circuit 4822 and functions as previouslydescribed in FIG. 48. This exemplary single up-conversion dual downconversion CATV tuner utilizes two PLLs that run off of a common 10 MHzcrystal oscillator 5302. From the 10 MHz crystal oscillator referencesthe PLLs generate two local oscillator signals that are used to mix downa received radio frequency to an intermediate frequency. This integratedCATV tuner advantageously uses differential signals throughout itsarchitecture to achieve superior noise rejection and reduced phasenoise. The receiver of the present invention advantageously provideschannel selectivity and image rejection on the chip to minimize thenoise injected into the received signal path. The differentialconfiguration also tends to suppress noise generated on the CMOSsubstrate as well as external noise that is radiated into thedifferential leads of the 10 MHz crystal that connect it to thesubstrate. In this embodiment, an external front end as previouslydescribed is supplied on a separate chip 5304 and an external filter5306 is utilized.

[0383] The details of integrated tuners are disclosed in more detail inU.S. patent application Ser. No. ______ filed ______ (B600:33756)entitled “Fully Integrated Tuner Architecture” by Pieter Vorenkamp,Klaas Bult, Frank Carr, Christopher M. Ward, Ralph Duncan, Tom W. Kwan,James Y. C. Chang and Haideh Khorramabadi; based on U.S. ProvisionalApplication No. 60/108,459 filed Nov. 12, 1998 (B600:33586), the subjectmatter of which is incorporated in this application in its entirety byreference.

Telephony Over Cable Embodiment

[0384]FIG. 54 is a block diagram of a low power embodiment of thereceiver that has been configured to receive cable telephony signals.These services among other cable services offered make use of RFreceivers. A cable telephone receiver converts an RF signals present onthe cable to a baseband signal suitable for processing to an audio, orother type of signal routed to a telephone system and a subscriber viatwo way transmission. When such services are widely offered, and arepackaged into a common device, per unit cost and power dissipation tendto become concerns. It is desirable to provide a low cost and powerefficient receiver.

[0385] Receivers integrated onto a single chip that incorporates filterson the chip reduce cost. However, placing filters onto a an integratedcircuit results in a high power consumption by the chip. On chip filtersrequire tuning circuitry that tends to consume significant amounts ofpower. Removal of this circuitry allows reduction of power levels tobelow 2 Watts per receiver. Each time that a signal is routed off of anintegrated circuit the chances of increasing system noise are increaseddue to the susceptibility of the external connections to the pick up ofnoise. Careful signal routing and the proper frequency planning of thepresent embodiment are calculated to reduce these undesired effects.

[0386] First, an input signal is passed through an RF front end chip5304 as previously described. The first frequency up conversion to thefirst IF 5402 is performed on the integrated receiver chip. Afterpassing a 50-860 MHz signal through a receiver front end 5304 thatprovides a differential output to the receiver chip 5404 the signal isdown converted to 1,220 MHz 5402. The 1,270 to 2,080 MHz LO 5406 isgenerated on chip by a first PLL circuit, PLL1 5408. The 1220 MHzdifferential signal is passed through buffer amplifiers 5410 and isapplied to an off chip differential signal filter 5412, with a centerfrequency at 1,220 MHz having a characteristic impedance of 200 Ohms.The differential signal tends to provide the necessary noise rejectionwhen routing the signal off and subsequently back onto the chip. Nextthe signal is routed back on to the integrated circuit 5404 where it isagain passed through a send buffer amplifier 5414.

[0387] The second frequency down conversion to the second IF 5416 isperformed on the integrated receiver chip. An 1,176 MHz differential Iand Q LO 5418 is generated on the integrated circuit by a second PLL,PLL2 5420 and polyphase 5422. The resulting second IF frequency 5616 is44 MHz. The mixer used to generate the second IF is an I/Q type mixer5424 that subsequently passes the signal through a polyphase circuit5426. The second IF is then passed through a third buffer amplifier5428. The signal is next routed off chip to a differential filtercentered at 44 MHz 5430. After filtering the signal is returned to theintegrated circuit where it undergoes amplification by a variable gainamplifier 5432.

[0388] The details of a low power receiver design are disclosed in moredetail in U.S. patent application Ser. No. ______ filed ______(B600:36232) entitled “System and Method for Providing a Low PowerReceiver Design” by Frank Carr and Pieter Vorenkamp; based on U.S.Provisional Application No. 60/159,726 filed Oct. 15, 1999 (B600:34672),the subject of which is incorporated in this application in its entiretyby reference.

Electronic Circuits Incorporating Embodiments of the Receiver

[0389]FIG. 55 shows a set top box 5502 used in receiving cabletelevision (CATV) signals. These boxes typically incorporate a receiver5504 and a descrambling unit 5506 to allow the subscriber to receivepremium programming. Additionally, on a pay for view basis subscriberscan order programming through their set top boxes. This functionadditionally requires modulation circuitry and a radio frequencytransmitter to transmit the signal over the CATV network 5508.

[0390] Set top boxes can, depending on the nature of the network,provide other services as well. These devices include, IP telephones,digital set-top cards that fit into PCs, modems that hook up to PCs,Internet TVs, and video conferencing systems.

[0391] The set-top box is the device that interfaces subscribers withthe network and lets them execute the applications that reside on thenetwork. Other devices in the home that may eventually connect with thenetwork include IP telephones, digital set-top cards that fit into PCs,modems that hook up to PCs, Internet TVs, and video conferencingsystems.

[0392] To satisfactorily provide digital services requiring highbandwidth, set top boxes must provide a easy to use interface betweenthe user and CATV provider. Memory 5510 and graphics driven by a CPU5512 tend to make the application as appealing as possible to a userwhen interfaced with a set top box 5514.

[0393] Also the set-top can receive data in Internet Protocol format andhas an IP address assigned to it. Also, satisfactory methods of handlingreverse path communications are required to provide interactive digitalservices. All of these services utilize an operating system resident inthe set top box 5502 for providing a user interface and communicatingwith the head end 5514 where the services are provided.

[0394] To receive services, and transmit requests for service,bidirectionally across a CATV network the data signal must be modulatedon a RF carrier signal. The set top box is a convenient place tomodulate the carrier for transmission, or to convert the modulatedcarrier to a base band signal for use at the user's location.

[0395] This is accomplished with a radio frequency (RF) transmitter andreceiver, commonly referred to in combination as a transceiver 5508. Abidirectional signal from a cable head end 5514 is transmitted over acable network that comprises cable and wireless data transmission. Atthe subscriber's location a signal 3406 is received an input to thesubscriber's set top box 5502. The signal 3406 is input to a set top boxtransceiver 5504. The set top box transceiver 5504 comprises one or morereceiver and transmitter circuits. The receiver circuits utilized areconstructed according to an embodiment of the invention. From the settop box transceiver, received data is passed to a decryption box 5506.If the television signal has been encrypted, this box performs anecessary descrambling operation on the signal. After being passedthrough the decryption box, the signal next is presented to a set topbox decoder 3416 where the signal is demodulated into audio and videooutputs 3414. The set top box incorporates a CPU 5512 with graphicscapabilities and a memory 5510 to provide an interface and control theset top box through a data transfer structure 5514. An optional inputoutput capability 5516 is provided for a direct user interface with theset top box. To transmit instructions from the user to the head end,information is transmitted over data transfer structure 5514 into thetransceiver module to the internal transmitter via the cable TV networkto the head end.

[0396]FIG. 56 is an illustration of the integrated television receiver5602. This television could be one that processes digital or analogbroadcast signals 5604. An exemplary integrated switchless attenuatorand low noise amplifier 3408 is the first stage in a receiver containedin a television set. The integrated switchless attenuator and low noiseamplifier is used as a “front end” of the receiver to adjust theamplitude of the incoming signal. Incoming television signals whetherreceived from a cable or antenna vary widely in strength, from receivedchannel to channel. Differences in signal strength are due to losses inthe transmission path, distance from the transmitter, or head end,obstructions in the signal path, among others.

[0397] The front end adjusts the received signal level to an optimumvalue. A signal that is too strong produces distortion in the subsequentcircuitry by over driving it into a non linear operating region. Asignal that is too week will be lost in the noise floor when subsequenthigh noise figure circuitry is used in an attempt to boost the signalstrength. When used in conjunction with “automatic level control” (5604)circuitry the integrated switchless attenuator and low noise amplifierresponds to a generated feed back signal input to its control voltageterminal to adjust the input signal level to provide optimumperformance.

[0398] After passing through the front end 3408, the RF signals 5604 areinput to tuner 5620. This tuner circuit is as described in the previousembodiments where a single channel is selected from a variety ofchannels presented in the input signal 5604. An automatic fine tuningcircuit (“AFT”) 4622 is provided to adjust the level of the final IFsignal 5624 being output to the television signal processing circuitry5610. The signal processing circuitry splits the audio signal 5602 offof the final IF signal 5624 and outputs it to an audio output circuitsuch as an amplifier and then to a speaker 5618. The video signal splitfrom IF signal 5624 is delivered via video signal 5606 to videoprocessing circuitry 5612. Here the analog or digital video signal isprocessed for application as control signals to the circuitry 5614 thatcontrols the generation of an image on a display device 5626. Such areceiver would typically be contained in a television set, a set topbox, a VCR, a cable modem, or any kind of tuner arrangement.

[0399]FIG. 57 is a block diagram of a VCR that incorporates anintegrated receiver embodiment 5702 in its circuitry. VCRs aremanufactured with connections that allow reception and conversion of atelevision broadcast signal 5704 to a video signal 5706. The broadcastsignals are demodulated 5708 in the VCR and recorded 5710 on a recordingmedia such as a tape, or output as a video signal directly. VCRs are acommodity item. Cost pressures require economical high performancecircuitry for these units to provide additional more features as theprices decline in the marketplace.

[0400]FIG. 58 shows a block diagram of a typical cable modem. A “CableModem” is a device that allows high speed data connection (such as tothe Internet) via a cable TV (CATV) network 5812. A cable modem commonlyhas two connections, one to the cable TV wall outlet 5802 and the otherto a computer 5804.

[0401] There are several methods for connecting cable modems tocomputers, Ethernet 10BaseT is an example. The coax cable 5808 connectsto the cable modem 5806, which in turn connects to an Ethernet card 5814in a PC. The function of the cable modem is to connect broadband (i.e.,the cable television network) to Ethernet. Once the Ethernet card hasbeen installed, the TCP/IP software is typically used to manage theconnection.

[0402] On-line access through cable modems allows PC users to downloadinformation at a speeds approximately 1,000 times faster than withtelephone modems. Cable modem speeds range from 500 Kbps to 10 Mbps.Typically, a cable modem sends and receives data in two slightlydifferent, or asynchronous fashions.

[0403] Data transmitted downstream, to the user, is digital datamodulated onto a typical 6 MHz channel on a television carrier, between42 MHz and 750 MHz. Two possible modulation techniques are QPSK(allowing data transmission of up to 10 Mbps) and QAM64 (allowing datatransmission of up to 36 Mbps). The data signal can be placed in a 6 MHzchannel adjacent to an existing TV signals without disturbing the cabletelevision video signals.

[0404] The upstream channel to the ISP provider is transmitted at a ratebetween 5 and 40 MHz. This transmission path tends to inject more noisethan the downstream path. Due to this problem, QPSK or a similarmodulation scheme in the upstream direction is desirable due to noiseimmunity above that available in other modulation schemes. However, QPSKis “slower” than QAM.

[0405] Cable modems can be configured to incorporate many desirablefeatures in addition to high speed. Cable modems can be configured toinclude, but are not limited to, a modem, a tuner 5816, anencryption/decryption device, a bridge, a router, a NIC card, SNMPagent, and an Ethernet hub.

[0406] To transmit and receive the data onto the cable televisionchannel it must be modulated and demodulated respectively. This isaccomplished with a radio frequency (RF) transmitter and receiver,commonly referred to in combination as a transceiver 5818. Thereceiver's front end 5820 is advantageously provided as previouslydescribed.

1. An automatic gain control circuit for a signal path comprising: aresistive network in the signal path having a plurality of taps thatrepresent different attenuations; an output circuit; means downstream ofthe output circuit for sensing the level of the signal in the signalpath; a plurality of amplifiers connecting the respective taps of theresistive network to the output circuit; and means responsive to thesensing means for controlling the amplifiers so only those tapsnecessary to compensate for changes in the sensed level of the signal inthe signal path are effectively connected across the output circuit. 2.The automatic gain control circuit of claim 1, in which the controllingmeans turns on fewer than all the plurality of amplifiers in a way thatsome of the amplifiers are fully turned on and some amplifiers arepartially turned on.
 3. The automatic gain control circuit of claim 2,in which the controlling means turns on different combinations ofamplifiers as the sensed level changes.
 4. The automatic gain controlcircuit of claim 3, in which the sensing means generates a controlsignal representative of the level of the signal in the signal path andthe controlling means additionally comprises a comparator correspondingto each amplifier, each comparator having first and second inputs, meansfor feeding different threshold signals to the first input of therespective comparators, means for feeding the control signal to thesecond input of the respective comparators, and means for connecting theoutputs of the respective comparators to the corresponding amplifiers sothe amplifiers are turned on by those comparators fed by a thresholdsignal exceeded by the control signal.
 5. An integrated receiver frontend comprising: a substrate; an attenuator disposed upon the substrate;and an amplifier disposed upon the substrate with its inputs coupled tothe attenuator output.
 6. The integrated receiver front end of claim 5wherein the substrate is silicon processed by a standard CMOS process.7. The integrated receiver front end of claim 5 wherein the attenuatoris a resistive ladder network.
 8. The integrated receiver front end ofclaim 5 wherein the resistive ladder network is implemented by combiningidentical valued resistors in series and parallel.
 9. The integratedreceiver front end of claim 5 wherein the attenuator comprises multipleattenuator stages each providing 1 dB of attenuation per stage.
 10. Theintegrated receiver front end of claim 9 wherein the attenuator stageseach comprise a 14.4 ohm series resistance and a 130 Ohm shuntresistance.
 11. The integrated receiver front end of claim 9 wherein theattenuator stages each comprise a chain of series connected resistorsconnected in parallel; whereby smaller steps in attenuation are providedat the interconnections of the chain of series connected resistors. 12.The integrated receiver front end of claim 5 wherein the amplifiercomprises a differential pair amplifier.
 13. The integrated receiverfront end of claim 12 wherein the amplifier comprises multipleamplifiers each connected to an attenuator output.
 14. The integratedreceiver front end of claim 13 wherein the amplifiers comprise: avariable current source; and a differential pair of transistors with afirst gate coupled to ground and a second gate coupled to the attenuatortap point, sources coupled to each other, and drains coupled to avariable current source.
 15. An integrated receiver front endcomprising: an attenuator that consists of a ladder network havingmultiple tap points, each providing a differing value of attenuation;and a differential pair amplifier connected to each tap point andselectively turned on, signally or as a group, to amplify the previouslyattenuated signal and provide a combined differential output with a lownoise figure.
 16. A method for processing a received radio frequencysignal comprising: receiving a radio frequency signal that is of varyingstrength; generating a control voltage to provide an indication of adesired signal strength; using the control voltage to turn on multiplecontiguous differential pair amplifiers that are connected to contiguoustap points on a resistive ladder network to allow an attenuated signalto be amplified after being attenuated to the desired attenuation level;passing the signal through the resistive ladder; tapping the resistiveladder at contiguous points allowing the signal to attenuated todifferent levels at each point; amplifying each signal by a fixed gainto produce a differential output; and combining the amplified signals toproduce a resultant signal of the desired signal strength.
 17. Anintegrated receiver front end comprising: a substrate means fordisposing a CMOS circuit; an attenuator means for reducing a signal'slevel disposed upon the substrate; an amplifier means for increasing asignal's level, disposed upon the substrate with its inputs coupled tothe attenuator output.